11/30/2011

11-30-11 - Basic sketch of Worker Thread system with dependencies

You have a bunch of worker threads and work items. Work items can be dependent, on other work items, or on external timed events (such as IO).

I've had some trouble with this for a while; I think I finally have a scheme that really works.

There are two queues :


RTR = ready to run : no dependencies, or dependencies are done

NR = not ready ; dependencies still pending

Each queue has an associated semaphore to count the number of items in it.

The basic work popping that each worker does is something like :


// get all available work without considering sleeping -
while( try_wait( RTR_sem ) )
{
    pop RTR_queue and do work
}

// (optionally spin a few times here and check RTR_sem)

// I may have to sleep -

wait( RTR_sem OR NR_sem ); // (*1)

if ( wakeup was from RTR_sem )
{
    pop RTR_queue and do work
}
else
{
    NRI (not ready item) = pop NR_queue
    deps = get dependencies that NRI needs to wait on

    wait( deps OR RTR_sem ); // (*3)

    if ( wakeup was from RTR_sem )
    {
        push NRI back on NR_queue and post NR_sem  // (*4)
        pop RTR_queue and do work
    }
    else
    {
        wakeup was because deps are now done
        NRI should be able to run now, so do it
        (*2)
    }  
}

*1 : the key primitive here is the ability to do a WFMO OR wait, and to know which one of the items signalled you. On Windows this is very easy, it's just WaitForMultipleObjects, which returns the guy who woke you. On other platforms it's trickier and probably involves rolling some of your own mechanisms.

Note that I'm assuming the semaphore Wait() will dec the semaphore at the time you get to run, and the OR wait on multiple semaphores will only dec one of them.

*2 : in practice you may get spurious wakeups or it may be hard to wait on all the dependencies, so you would loop and recheck the deps and possibly wait on them again.

How this differs from my previous system :

My previous system was more of a traditional "work stealing" scheme where each worker had its own queue and would try to just push & pop works from its own queue. This was lower overhead in the fast path (it avoids having a single shared semaphore that they have to contend on, for example), but it had a lot of problems.

Getting workers to go to sleep & wake up correctly in a work stealing scheme is a real mess. It's very hard to tell when you have no work to do, or when you have enough work that you need to wake a new worker, because you don't atomically maintain a work count (eg. a semaphore). You could fix this by making an atomic pair { work items, workers awake } and CAS that pair to maintain it, but that's just a messy way of being a semaphore.

The other problem was what happens when you have dependent work. You want a worker to go to sleep on the dependency, so that it yeilds CPU time, but wakes up when it can run. I had that, but then you have the problem that if somebody else pushes work that can immediately run, you want to interrupt that wait on the dependency and let the worker do the ready work. The semaphore OR wait fixes this nicely.

If you're writing a fractal renderer or some such nonsense then maybe you want to make lots of little work items and have minimal overhead. But that's a very special purpose rare case. Most of the time it's much more important that you do the right work when possible. My guiding principles are :

If there is no work that can be done now, workers should go to sleep (yield CPU)
If there is work that can be done now, workers should wake up
You should not wake up a worker and have it go back to sleep immediately
You should not have work available to do but the workers sleeping

Even in the "fractal renderer" sort of case, where you have tons of non-dependent work items, the only penalty here is one extra semaphore dec per item, and that's just a CAS (or a fetch_add) assuming you use something like "fastsemaphore" to fast-path the case of being not near zero count.

There is one remaining issue, which is when there is no ready-to-run work, and the workers are asleep on the first semaphore (they have no work items). Then you push a work item with dependencies. What will happen in the code sketch above is that the worker will wake up, pop the not ready item, then go back to sleep on the dependency. This violates article 3 of the resolution ("You should not wake up a worker and have it go back to sleep immediately").

Basically from *1 to *3 in the code is a very short path that wakes from one wait and goes into another wait; that's always bad.

But this can be fixed. What you need is "wait morphing". When you push a not-ready work item and you go into the semaphore code that is incrementing the NR_sem , and you see that you will be waking a thread - before you wake it up, you take it out of the NR_sem wait list, and put it into the NRI's dependency wait list. (you leave it waiting on RTR_sem).

That is, you just leave the thread asleep, you don't signal it to wake it up, it stays waiting on the same handle, but you move the handle from NR_sem to the dependency. You can implement this a few ways. I believe it could be done with Linux'es newer versions of futex which provide wait morphing. You would have to build your semaphore and your dependency waiting on futex, which is easy to do, then wait morph to transfer the wait. Alternatively if you build them on "waitset" you simply need to move an item from one waitset to the other. This can be done easily if your waitset uses a mutex to protect its internals, you simply lock both mutexes and move the waitable handle with both held.

The net result with wait morphing is very nice. Say for example are you workers are asleep. You create a work item that is dependent on an IO and push it. None of the workers get woken up, but one of them is changed from waiting on work available to waiting on the dependency. When the IO completes it wakes that worker and he runs. If somebody pushed a ton of work in the mean time, all the workers would be woken and they would do that work, and the dependent work would be pushed back on the NR queue and set aside while they did RTR work.

ADDENDUM : at the spot marked (*4) :


push NRI back on NR_queue and post NR_sem // (*4)
pop RTR_queue and do work

In real code you need do something a bit more complex here. What you do is something like :

if ( NRI is ready ) // double check
{
  RTR_sem.post() // we woke from RTR_sem , put it back
  do NRI work
}
else
{
  push NRI onto NR_lifo and post NR_sem
  pop RTR_queue and do work
}

we've introduced a new queue , the NR_lifo which is a LIFO (eg. stack). Now whenever you get an NR_sem post, you do :

// NR_sem just returned from wait so I know an NR item is available :

NRI = NR_lifo.pop()
if ( NRI == NULL )
  NRI = NR_queue.pop()

the item must be in one or the other and we prefer to take from the LIFO first. Basically the LIFO is a holding area for items that were popped off the FIFO and were not yet ready, so we want to keep trying to run those before we go back to the FIFO. You can use a single semaphore to indicate that there is an item in either queue.

11-30-11 - Some more Waitset notes

The classic waitset pattern :

check condition

waiter w;
waitset.prepare_wait(&w);

double check condition

w.wait();

waitset.retire_wait(&w);

lends itself very easily to setting a waiter flag. All you do is change the double check into a CAS that sets that flag. For example say your condition is count > 0 , you do :

if ( (count&0x7FFFFFFF) == 0 )
{
    waiter w;
    waitset.prepare_wait(&w);

    // double check condition :
    int c = count.fetch_or( 0x80000000 ); // set waiter flag and double check
    if ( (c&0x7FFFFFFF) == 0 )
        w.wait();

    waitset.retire_wait(&w);
}

then in notify, you can avoid signalling when the waiter flag is not set :

// publish :
int c = count.atomic_inc_and_mask(1,0x7FFFFFFF);
// notify about my publication if there were waiters :
if ( c & 0x80000000 )
  waitset.notify();

(note : don't be misled by using count here; this is still not a good way to build a semaphore; I'm just using an int count as a simple way of modeling a publish/consume.


I was being obtuse before when I wrote about the problems with waitset OR. It is important to be aware of those issues when working with waitsets, because they are inherent to how waitsets work and you will encounter them in some form or other, but of course you can do an OR if you extend the basic waitset a little.

What you do is give waiter an atomic bool to know if it's been signalled, something like :


struct waiter
{
  atomic<bool> signalled;
  os_handle  waitable_handle;
}

(a "waiter" is a helper which is how you add your "self" to the waitset; depending on the waitset implementation, waitable_handle might be your thread ID for example).

Then in the waitset notify you just do :


if ( w->signalled.exchange(true) == false )
{
   Signal( w->waitable_handle );
}
else
    step to next waiter in waitset and try him again.

That is, you try to only send the signal to handles that need it.

If we use this in the simple OR example from a few days ago, then both waiting threads will wake up - two notify_ones will wake two waiters.

While you're at it, your waiter struct may as well also contain the origin of the signal, like :


if ( w->signalled.exchange(true) == false )
{
    // non-atomic assignment :
    w->signal_origin = this; // this is a waitset
    Signal( w->waitable_handle );
}

That way when you wake from an OR wait you know why.

(note that I'm assuming your os_handle only ever does one state transition - it goes from unsignalled to signalled. This is the correct way to use waitset; each waiter() gets a new waitable handle for its lifetime, and it only lives for the length of one wait. In practice you actually recycle the waiters to avoid creating new ones all the time, but you recycle them safely in a way that you know they cannot be still in use by any thread (alternatively you could just have a waiter per thread in its TLS and reset them between uses))

(BTW of course you don't actually use atomic bool in real code because bool is too badly defined)

11/28/2011

11-28-11 - Some lock-free rambling

It helps me a lot to write this stuff down, so here we go.

I continually find that #StoreLoad scenarios are confusing and catch me out. Acquire (#LoadLoad) and Release (#StoreStore) are very intuitive, but #StoreLoad is not. I think I've covered almost this exact situation again, but this stuff is difficult so it's worth revisiting many times. (I find low level threading to be cognitively a lot like quantum mechanics, in that if you do it a lot you become totally comfortable with it, but if you stop doing it even for a month it is super confusing and bizarre when you come back to it, and you have to re-work through all the basics to convince yourself they are true).

(Aside : fucking Google verbatim won't even search for "#StoreLoad" right. Anybody know a web search that is actually verbatim? A whole-word-only option would be nice too, and also a match case option. You know, like basic text search options from like 1970 or so).

The classic case for needing #StoreLoad is WFMO. The very simple scenario goes like this :


bool done1 = false;
bool done2 = false;

// I want to do X() when done1 & done2 are both set.

Thread1:

done1 = true;
if ( done1 && done2 )
    X();

Thread2:

done2 = true;
if ( done1 && done2 )
    X();

This doesn't work.

Obviously Thread1 and Thread2 can run in different orders so done1 and done2 become set in random order. But one thread or the other should see them both set. But they don't; the reason is that the memory visibility can be reordered. This is a pretty clear illustration of the thing that trips up many people - threads can interleave both in execution order and in memory visibility order.

In particular the bad execution case goes like this :


done1 = false, done2 = false

T1 sets done1 = true
  T1 sees done1 = true (of course)
  T2 still sees done1 = false (store is not yet visible to him)

T2 sets done2 = true
  T2 sees done2 = true
  T1 still sees done2 = false

T1 checks done2 for (done1 && done2)
  still sees done2 = false
  doesn't call X()

T2 checks done1
  still sees done1 = false
  doesn't call X()

later
T1 sees done2=true
T2 sees done1=true

when you write it out it's obvious that the issue is the store visibility is not forced to occur before the load. So you can fix it with :

Thread1:

done1 = true;
#StoreLoad
if ( done1 && done2 )
    X();

As noted previously there is no nice way to make a StoreLoad barrier in C++0x. The best method I've found is to make the loads into fetch_add(0,acq_rel) ; that works by making the loads also be stores and using a #StoreStore barrier to get store ordering. (UPDATE : using atomic_thread_fence(seq_cst) also works).


The classic simple waitset that we have discussed previously is a bit difficult to use in more complex ways.

Refresher : A waitset works with a double-check pattern, like :


signalling thread :

set condition
waitset.notify();

waiting thread :

if ( ! condition )
{
    waitset.prepare_wait()

    // double check :
    if ( condition )
    {
        waitset.cancel_wait();
    }
    else
    {
        waitset.wait();
    }
}

we've seen in the past how you can easily build a condition var or an eventcount from waitset. In some sense waitset is a very low level primitive and handy for building higher level primitives from. Now on to new material.

You can easily use waitset to perform an "OR" WFMO. You simply add yourself to multiple waitsets. (you need a certain type of waitset for this which lets you pass in the primitive that you want to use for waiting). To do this we slightly extend the waitset API. The cleanest way is something like this :


instead of prepare_wait :

waiter create_waiter();
void add_waiter( waiter & w );

instead of wait/cancel_wait :

~waiter() does cancel/retire wait 
waiter.wait() does wait :

Then an OR wait is something like this :

signal thread 1 :

set condition1
waitset1.notify();

signal thread 2 :

set condition2
wiatset2.notify();


waiting thread :

if ( condition1 ) // don't wait

waiter w = waitset1.create_waiter();

// double check condition1 and first check condition2 :

if ( condition1 || condition2 ) // don't wait
  // ~w will take you out of waitset1

waitset2.add_waiter(w);

// double check :

if ( condition2 ) // don't wait

// I'm now in both waitset1 and waitset2
w.wait();

Okay. This works fine. But there is a limitation which might not be entirely obvious.

I have intentionally not made it clear if the notify() in the signalling threads is a notify_one (signal) or notify_all (broadcast). Say you want it to be just notify_one , because you don't want to wake more threads than you need to. Say you have this scenario :


X = false;
Y = false;

Thread1:
X = true;
waitsetX.notify_one();

Thread2:
Y = true;
waitsetY.notify_one();

Thread3:
wait for X || Y

Thread4:
wait for X || Y

this is a deadlock. The problem is that both of the waiter threads can go to sleep, but the two notifies might both go to the same thread.

This is a general difficult problem with waitset and is why you generally have to use broadcast (for example eventcount is built on waitset broadcasting).

You may think this is an anomaly of trying to abuse waitset to do an OR, but it's quite common. For example you might try to do something seemingly simple like build semaphore from waitset.


class semaphore_from_waitset
{
    waitset_simple m_waitset;
    std::atomic<int> m_count;

public:
    semaphore_from_waitset(int count = 0)
    :   m_count(count), m_waitset()
    {
        RL_ASSERT(count >= 0);
    }

    ~semaphore_from_waitset()
    {
    }

public:
    void post()
    {
        m_count($).fetch_add(1,mo_acq_rel);
        // broadcast or signal :
        // (*1)
        //m_waitset.notify_all();
        m_waitset.notify_one();
    }

    bool try_wait()
    {
        // see if we can dec count before preparing the wait
        int c = m_count($).load(mo_acquire);
        while ( c > 0 )
        {
            if ( m_count($).compare_exchange_weak(c,c-1,mo_acq_rel) )
                return true;
            // c was reloaded
        }
        return false;
    }

    void wait(HANDLE h)
    {
        for(;;)
        {
            if ( try_wait() )
                return;
    
            // no count available, get ready to wait
            ResetEvent(h);
            m_waitset.prepare_wait(h);
            
            // double check :
            if ( try_wait() )
            {
                m_waitset.retire_wait(h);
                // (*2)
                // pass on the notify :
                m_waitset.notify_one();
                return;
            }
            
            m_waitset.wait(h);
            m_waitset.retire_wait(h);
            // loop and try again
        }
    }
};

it's totally straightforward in the waitset pattern, except for the broadcast issue. If *1 is just a notify_one, then at *2 you must pass on the notify. Alternatively if you don't have the re-signal at *2 then the notify at *1 must be a broadcast (notify_all).

Now obviously if you have 10 threads waiting on a semaphore and you inc the count by 1, you don't want all 10 threads to wake up so that just 1 of them can dec the count and get to execute. The re-signal method will wake 2 threads, so it's better than broadcast, but still not awesome.

(note that this is easy to fix if you just put a mutex around the whole thing; or you can implement semaphore without waitset; the point is not to reimplement semaphore in a bone-headed way, the point is just that even very simple uses of waitset can break if you use notify_one instead of notify_all).

BTW the failure case for semaphore_from_waitset with only a notify_one and no resignal (eg. if you get the (*1) and (*2) points wrong) goes like this :


the problem case goes like this :

    T1 : sem.post , sem.post
    T2&T3 : sem.wait

    execution like this :

    T2&3 both check count and see zereo
    T1 now does one inc and notify, noone to notify yet
    T2&3 do prepare_wait
    T2 does its double-check, sees a count and takes it (does not retire yet)
    T3 does its double-check, sees zero, and goes to sleep
    T1 now does the next inc and notify
    -> this is the key problem
    T2 can get the notify because it is still in the waiter list
        (not retired yet)
    but T3 needs the notify

The key point is this spot :

            // double check :
            if ( try_wait() )
            {
                // !! key !!
                m_waitset.retire_wait(h);

you have passed the double check and are not going to wait, but you are still in the waiter list. This means you can be the one thread chosen to receive the signal, but you don't need it. This is why resignal works.

11/23/2011

11-23-11 - This is not okay

Fuck this shit. I'm going to Hawaii.

11/22/2011

11-22-11 - The Mature Programmer

1. The Mature Programmer

The mature programmer manages their own time and productivity well. The MP knows that maintenance is as much work as the initial writing and code always takes longer than you think. The MP knows that any changes to code can introduce bugs, no matter how seemingly trivial. The MP knows that premature optimization is foolish and dangerous. The MP knows that sexy coding like writing big complex systems from scratch is rarely the best way to go. The MP does not get into ego competitions about who has the prettiest code. The MP acheives the best final result in the minimum amount of time.

When I started at Oddworld, I was watching lots of game companies get into dick-waving contests about who had the greatest home-rolled graphics engine, and I was also watching lots of indie developers spend massive amounts of time on their "one true" product and never actually ship it. I resolved that we would not fall into those traps - we would be humble and not reach too far, we would not let our egos stop us from licensing code or using old fashioned solutions to problems, we would stay focused on the end product - any sexy code that didn't produce a visible benefit in the actual shipping game was nixed. For the most part I think we succeeded in that (there were a few digressions that were mainly due to me).

But the way of the Mature Programmer can be a trap which comes back to bite you.

The problem is that writing code in this way is not very much fun. Sure there's the fun of making the product - and if you're working on a game and believe in the game and the team, then just seeing the good product come out can give you motivation. But if you don't have that, it can be a real slog.

Most of us got into programming not for the end products that we create, but because the programming itself is a joy. Code can be beautiful. Code can be a clever, artistic, exciting creation, like a good mathematical proof. The Mature Programmer would say that "clever code is almost always dangerous code". But fuck him. The problem is that when you get carried away with being "mature" you suck the joy right out coding.

You need to allow yourself a certain amount of indescretions to keep yourself happy with your code. Sure those templates might not actually be a good idea, but you enjoy writing the code that way - fine, do it. Yes, you are optimizing early and it just makes the code harder to maintain and harder to read and more buggy - but you love to do that, fine, do it.

Obviously you can't go overboard with this, but I think that I (and many others) have gone overboard with being mature. Basically in the last ten years of my evolution as a coder I have become less of a wild card "hot shot" and more of a productivity manager, an efficient task analyzer and proactive coordinater of code-actualizing solutions. It's like a management beaurocracy of one inside my head. It's horrible.

I think there are two factors to consider : first is that being "mature" and productive can cause burnout which winds up hurting your productivity, or it can just make coding unpleasant so you spend fewer hours at it. Most "mature" coders brag about the fact that they can get as much done in 6 hours as they used to do in 14. But those 14 hours were FUN, you coded that long because you loved it, you couldn't get to sleep at night because you wanted to code more; now the 6 hours is all sort of unpleasant because instead of rolling your own solution you're just tying together some java and perl packages. Second is that being productive is not the only goal. We are coding to get some task done and to make money, but we're also coding because we enjoy it, and actually being less productive but enjoying your coding more may be a net +EV.

2. The healthy opposition of a producer

Many programmers in normal coding jobs hate having the interference of a producer (or corporate management, or the publisher, or whatever). This person enforces stupid schedules and won't let us do the features we want, and urrgh we hate them! These coders long to be able to make their own schedules and choose their own tasks and be free.

It's actually a very healthy and much more relaxing in many ways to have that opposition. When you have to schedule yourself or make your own decisions about tasks, you become responsible for both the creative "reach for the sky" side and the responsible "stay in budget" side. It's almost impossible to do a good job of both sides. This can happen if you are an indie or even if you are a powerful lead with a weak producer.

Most creative industries know that there is a healthy opposition in having the unconscrained creative dreamer vs. the budget-enforcing producer. You don't want the dreamer to be too worried about thinking about schedules or what's possible - you just want them to make ideas and push hard to get more.

When you have to cut features or crunch or whatever, it's nice to have that come from outside - some other force makes you do it and you can hate them and get on with it. It's nice to have that external force to blame that's not on your team; it gives you a target of your frustration, helps bonding, and also gives you an excuse to get the job done (because they told you to).

When you have to balance dreams vs schedules on your own, it adds an intellectual burden to every task - as you do each task you have to consider "is this worth the time? is this the right task to do now? should I do a simpler version of this?" which greatly reduces your ability to focus just on the task itself.

3. Coding standards

It's kind of amazing to me how many experienced programmers still just don't understand programming. The big difficulty in programming is that the space of the ways to write something are too large. We can get lost in that space.

One of the problems is simply the intellectual overload. Smart coders can mistakenly believe that they can handle it, but it is a burden on everyone. Every time you write a line of code, if you have to think "should I use lower case or mixed caps?" , or "should I make this a function or just write it in line?" , your brain is spending masses of energy on syntactic decisions and doesn't have its full power for the functionality. Strict coding standards are actually an intellectual relief because they remove all those decisions and give you a specific way to do the syntax. (The same of course goes for reading other people's code - your eyes can immediately start looking at the functionality, not try to figure out the current syntax)

The other big benefit of coding standards is creating a "meta language" which is smaller than the parent language and enforces certain invariants. By doing that you again reduce the space that the brain has to consider. For example you might require that all C macros behave like functions (eg. don't eat scopes and don't declare variables). Now when I see one I know I don't have to worry about those things. Or you might require that globals are never externed and only get accessed through functions called "GetGlobal_blah". It doesn't really matter what they are as long as they are simple, clear, uniform, and strictly enforced, because only if they are totally reliable can you stop thinking about them.

4. The trap of "post-Mature Programmer" ism.

Many great coders of my generation have gone through the strict clean rules-following coder phase and have moved onto the "post" phase. The "post-mature programmer" knows the importance of following strict coding style rules or not indulging themselves too much, but also sees the benefit of bending those rules and believes that they can be a bit more free about deciding on what to do for each situation.

I believe that they/we mostly get this wrong.

The best analogy I can think of is poker. Most successful poker players go through several phases. First you think you're ever so clever and you can bluff and trap people and play all sorts of weird lines. Once you move up levels and start playing serious poker this delusion is quickly wiped out and you realize you need to go back to fundemantals. So then most people will go through the TAG "standard line" phase where they learn the right thing to do in each situation and the standard way to analyze hands, and they will be quite successful with this. (note that "standard line" doesn't mean nitty, it involves things like squeeze plays and even check-shove river bluffs, but it's based on playing a balanced range and studying EV). But then they are so successful with their solid play that they start to think they can get away with "mixing it up", playing hands that are almost certainly not profitable because they think they are good enough post-flop to make up for it (eg. Durrr style), or imagining that by playing some minus EV hands it helps their image and pays off later.

This is almost always wrong. Limping AA is almost always wrong, opening 72o UTG is almost always wrong - maybe you've done some analysis and you've decided it's the right thing at this table at this moment (for example limping AA because the people behind you attack limpers way too much and they think you would never limp AA so they will get stuck easily). It's wrong.

(telling yourself that your current bad play is made up for with later "image value" is one of the great rationalizations that poker players use an excuse to justify their bad play. programmers due to same with a set of excuses like "performance" that are really just rationalizing justifications for their bad practices; with poker, EV in the hand is worth more than EV in the bush; that is, the later image value you might win is so small and dubious and depends on various things working out just right that it's almost never correct to give up known current value for possible future value. (a particularly simple case of this is "implied odds" which bad players use an excuse to chase hands they shouldn't))

The problem is that when you open yourself up to making any possible move at any moment, there is simply too much to consider. You can't possibly go through all those decisions from first principles and make the right choice. Even if you could, there's no way you can sustain it for thousands of hands. You're going to make mistakes.

The same is true in coding; the post-MP knows the value of encapsulating a bit of functionality into a struct + helpers (or a class), but they think I'm smart enough I can decide not to do that in this particular case. No! You are wrong. I mean, maybe you are in fact right in this particular case, but it's not a good use of your brain energy to make that decision, and you will make it wrong some times.

There is a great value in having simple rules. Like "any time I enter a pot preflop, I come in for a raise". It may not always be the best thing to do, but it's not bad, and it saves you from making possibly big mistakes, and most importantly it frees up your brain for other things.

The same thing happens with life decision making. There's a standard set of cliches :

Don't marry the first person you sleep with
Don't get in a serious relationship off a rebound
Don't buy anything if the salesman is pushing it really hard
Take a day to sleep on any big decision
Don't lend money to poor friends
etc.
you may think "I'm smart, I'm mature, I don't need these rules, I can make my own decision correctly based on the specifics of the current situation". But you are wrong. Sure, following the rules you might miss out on the truly optimum decision once in a while. But it's foolish arrogance to think that your mind is so strong that you don't need the protection and simplicity that the rules provide.

In poker the correct post-solid-player adjustment is very very small. You don't go off making wild plays all the time, that's over-confidence in your abilities and just "spew". A correctly evolved player basically sticks to the solid line and the standard way of evaluating, but knows how to indentify situations where a very small correction is correct. Maybe the table is playing too tight preflop, so in the hijack position you start opening the top 35% of hands instead of the top 25% of hands. You don't just start opening every hand. You stay within the scope of the good play that you understand and can do without rethinking your whole approach.

The same is true in programming I believe; the correct adjustment for post-mature coding is very small; you don't have to be totally dogmatic about making every member variable private, but you also don't just stop encapsulating classes at all.

11/09/2011

11-09-11 - Weird shite about Exceptions in Windows

What happens when an exception is thrown in Windows ? (please fill in any gaps, I haven't researched this in great detail).

1. The VectoredExceptionHandlers are called. One of these you may not be aware of is the "first chance" exception handler that the MSVC debugger installs. If you have the flags set in a certain way, this will cause you to breakpoint at the spot of the throw without passing the exception on to the SEH chain.

2. The list of __except() handlers is walked and those filters are invoked; if the filter takes the exception then they handle it.

* of note here is the change from x86 to x64. Under x86 SEH handlers were made on the stack and then tacked onto the list as you descended (basically the __try corresponds to tacking on the handler); under x64 that is all removed and the SEH filter walk relies on being able to trace back up the function call stack. Normally there's no difference, however under x64 if your function call stack can't be walked for some reason, then your SEH filters won't get called! This can happen for a few reasons; it can happen due to the 32-64 thunk layer, it can happen if you manually create some ASM or "naked" functions and don't maintain the stack trace info correctly, and it can happen of course if you stomp the return addresses in the stack. See for example : The case of the disappearing OnLoad exception � user-mode callback exceptions in x64 at Thursday Night . (stomping the stack can of course ruin SEH on x86 as well since the exception registration structures are on the stack).

More info on the x64 SEH trace : at osronline and at nynaeve .

3. If no filter wanted the exception, it goes up to the UnhandledExceptionFilter. In MSVC's CRT this is normally set to __CxxUnhandledExceptionFilter, that function itself will check if a debugger is present and do different things (eg. breakpoint).

4. If UnhandledExceptionFilter still didn't handle the exception and passes it on, the OS gets it and you get the application critical error popup box. Depending on your registry settings this may invoke automatic debugging. However as noted here : SetUnhandledExceptionFilter and VC8 - Jochen Kalmbach's WebLog there is a funny bypass where the OS will pass the exception directly to the OS handler and not call your filter.

Automatic debugging is controlled by

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug].  
when it was first introduced it defaulted to Dr Watson. At some point (Vista?) that was changed to point it to the Windows Troubleshooting engine instead. I believe that when you install DevStudio this registry key is changed to point to vsjitdebugger. The key "Auto" is set to 0 by default which means ask before popping into the debugger.

To clarify a bit about what happens with unhandled exceptions : your unhandled exception callback is not called first, and is not necessarily called at all. After all the SEH filters get their chance, the OS calls its own internal "UnhandledExceptionFilter" - not your callback. This OS function checks if you are in a debugger and might just pass off the exception to the debugger (this is *not* the "first chance" check which is done based on your MSVC check boxes). This function also might just decide that the exception is a security risk and pass it straight to the AeDebug. If none of those things happen, then your filter may get called. (this is where the CRT CxxUnhandledExceptionFilter would get called if you didn't install anything).

Another note : the standard application error popup box just comes from UnhandledExceptionFilter. One of the ways you can get a silent application exit with no popup is if the OS detects that your SEH chain is corrupted, it will just TerminateProcess on your ass and drop you out. Similarly if you do something bad from inside one of your exception handlers. (another way you can get a silent TerminateProcess is if you touch things during thread or process destruction; eg. from a DLL_THREAD_DETACH or something like that, if you try to enter crit secs that are being destroyed you can get a sudden silent process exit).


Some links :

DebugInfo.com - Unexpected user breakpoint in NTDLL.DLL
Under the Hood New Vectored Exception Handling in Windows XP
SetUnhandledExceptionFilter Anti Debug Trick � Evilcodecave�s Weblog
SetUnhandledExceptionFilter and VC8 - Jochen Kalmbach's WebLog
SetErrorMode function
C++ tips AddVectoredExceptionHandler, AddVectoredContinueHandler and SetUnhandledExceptionFilter - Zhanli's tech notes - Sit
A Crash Course on theDepths of Win32 Structured Exception Handling, MSJ January 1997

A bit of interesting stuff about how the /RTC run time checks are implemented :

Visual C++ Debug Builds��Fast Checks� Cause 5x Slowdowns Random ASCII

A bit about stack sizes on windows, in particular there are *two* thread stack sizes (the reserved and initial commit) and people don't usually think about that carefully when they pass a StackSize to CreateThread :

Thread Stack Size

Not directly related but interesting :

Pushing the Limits of Windows Processes and Threads - Mark's Blog - Site Home - TechNet Blogs
Postmortem Debugging Dr Dobb's
John Robbins' Blog How to Capture a Minidump Let Me Count the Ways
Collecting User-Mode Dumps
Automatically Capturing a Dump When a Process Crashes - .NET Blog - Site Home - MSDN Blogs

11/08/2011

11-08-11 - Differences Running in Debugger

Bugs that won't repro under debugging are the worst. I'm not talking about "debug" vs "release" builds; I mean the exact same exe, run in the debugger vs. not in the debugger.

What I'd like is to assemble a list of the differences between running under the debugger and not under the debugger. I don't really know the answers to this so this is a community participation post. eg. you fill in the blanks.

Differences in running under the debugger :

1. Timing. A common problem now with heavily threaded apps are bugs due to timing variation. But where do the timing differences come from exactly?

1.a. OutputDebugString. Duh, affects timing massively. Similarly anything you do dependent on IsDebuggerPresent().

1.b. VC-generated messages about thread creation etc. These obviously affect timing. You can disable them being shown by right-clicking in the output window of the debugger, but the notification is still being sent so you can never completely eliminate the timing difference for creating/destroying threads. (and the debugger does a lot more work for thread accounting anyway, so create/destroy will always have significant timing variation).

2. Exceptions. (not C++ exceptions, which are handled pretty uniformly, but more the low level SEH exceptions like access violations and such). Obviously in the debugger you can toggle the handling of various exceptions and that can change behavior. One thing I'm not sure of is if there are any registry settings or other variables that control exception behavior in NON-debugged runs? (* more on this in another post)

3. Stack. Is there a difference here? Not that I know of.

4. Debug Heap. This is probably the biggest one. Processes run in the debugger on windows *always* get the debug heap, even if you didn't ask for it. You can turn this off by setting _NO_DEBUG_HEAP as an environment variable or starting MSVC with -hd. See Behavior of Spawned Processes .

Note that this isn't coming from MSVC, it's actually in ntdll. When you create your process heap, ntdll does a "QueryInformationProcess" and sees if it's being debugged, and if so it stuffs in the debug heap. The important thing is that this is at heap creation time, which leads to a solution.

5. Child Process issues. Because the debugged process is a child process of the debugger, it inherits its process properties. (the same issue can occur for running under "cmd" vs. spawning from explorer). Two specifics are "permissions" and environment variables. Another inherited value is the "ErrorMode" as in "GetErrorMode/SetErrorMode".

There's a solution to #4 and #5 which is this :

Start your app outside of the debugger. Make it do an int 3 so it pauses. Then attach the debugger. You can now debug bug you don't get some of the ran-from-debugger differences.

(note to self about attaching : for some reason the MSVC "attach to running process" seems to fail a lot; there are other ways to do it though, when you get an int 3 message box popup you can click "debug" there, or from task manager or procexp you can find the task and click "debug" there).

11/03/2011

11-03-11 - The difficulty of school reform

I'm so opposed to top-down metric based "reform" that I figured I should talk about what I think is a better alternative.

First of all there is no doubt that American public schools are sick. There are lots of good teachers and good classes, but also lots of bad. In my opinion, we don't need massive structural reform, we need a way to get rid of the bad teachers.

(almost always a cluster of bad teachers goes with a bad principal and often a bad superindendent too; they tend to be teachers with seniority who just don't care much anymore, and they all just want to maintain the status quo)

I do believe that charters are not the answer. There's nothing wrong with private schools, but they should be private. I don't believe that federal money should go to private institutions, almost ever, because it leads to corruption, and it also just sucks funding out of the public school system. The charters almost always wind up being a way to discriminate about entrants in some way (even just by desire to go to them), and are also often just a way to sneak around the teacher's union. Anyhoo.

I think the answer is motivating teachers and rewarding good teachers, and also being able to fire bad teachers. If teachers are motivated to succeed, and principals are motivated to hire good teachers and fire bad ones, you have a more free labor market and things will improve.

But how do you do that? This is where the trouble comes in.

I believe standardized test performance is a terrible way to do measure success. Most simple metrics like this would be similarly bad.

Judgement by a panel of peers doesn't work, because the teachers get into collusion and just say everyone is great. Perhaps this could be improved by making teachers grade each other, and forcing the grade to be on a curve so there are gauranteed to be winners and losers. But this would just degenerate into a game of "Survivor" where the old guard makes alliances to vote for each other and so on.

I believe the best answer is to let parents grade the teachers. Schools are one of the few areas where I think local government is actually better than top-down federal government, because it's one of the few areas where the local people actually pay attention to what's happening and get involved. (on the other hand, I think local school funding is probably unconstitutional and needs to be abolished; it creates great inequality to this day, despite many court rulings trying to redistribute funding (such as the "robin hood" ruling in Texas))

One idea is to let parents apply for what school they want their kids in and what specific teacher they want. Kids are then assigned by lottery, but you count the number of applications each teacher gets and that's their score. It's basically measuring demand as if teaching was a good. Teachers with high scores get raises and teachers with low scores get fired.

Now you obviously have to control for things like teachers just giving all A's, so people apply because it's the "easy" teacher. One solution might to force all classes to be graded on a bell. That would actually balance out the social stratification of classes because the grade-grubber kids might want to avoid the most prestigious classes (since they would be full of smart kids and very hard to do well on with a bell curve).

That's all sort of okay I think, but there's a big problem, which is that it biases strongly against areas where the parents don't give a shit. And those are the most problematic areas.

11-03-11 - BoolYouMustCheck

I've got a lot of functions that return error codes (rather than throw or something). The problem with that is that it's very easy to just not check the error code and then you have incorrect code that can possibly break in a nasty way if the error case is hit and not detected.

One way to test this is like this :


class BoolYouMustCheck
{
private:
    bool m_b;
    mutable bool m_checked;

public :

    //BoolYouMustCheck() : m_b(false), m_checked(false) { }
    BoolYouMustCheck(bool b) : m_b(b), m_checked(false) { }
    
    ~BoolYouMustCheck()
    {
        ASSERT( m_checked );
    }
    
    operator bool () const
    {
        m_checked = true;
        return m_b;
    }

};

it's just a proxy for bool which will assert if it is assigned and never read.

So now you can take a function that returns an error condition, for example :


bool func1(int x)
{
    return ( x > 7 );
}

normally you could easily just call func1() and not check the value. But you change it to :

BoolYouMustCheck func2(int x)
{
    return ( x > 7 );
}

(in practice you probably just want to do #define bool BoolYouMustCheck)

Now you get :


{

    int y = clock();

    // asserts:
    func1(y);

    // asserts :
    bool b1 = func1(y);
    
    // okay :
    bool b2 = func1(y);
    if ( b2 )
        y++;
    
    // okay :
    if ( func1(y) )
        y++;
        
    return y;
}

which is kind of nice.

The only ugly thing is that the assert can be rather far removed from the line of code that caused the problem. In the first case (just calling func1 and doing nothing with the return value), you get an assert right away, because the returned class is destructed right away. But in the second case where you assign to b1, you don't get the assert until the end of function scope. I guess you could fix that by taking a stack trace in the constructor.

(note : if you want to intentionally ignore the return value b1 you can just add a line like (int) b1; to surpress the assert.

11/02/2011

11-02-11 - StringMatchTest Release

Code for my string match testbed discussed previously. I'm not gonna do the work to turn this into a clean standalone, so it's a big mess and you can take what you like out of it.

stringmatchtest.zip (45k)

Note : the stringmatchtest.vcproj project refers to some files that are not included in this distribution. Just delete them from the project.

Requires cblib.zip (633k)

You may also need STLPort (I haven't tried building with the VC STL , I use STLPort 5.1.5 or 5.2.1). (BTW I had to modify the STLPort headers to make it build on VS 2008 ; the mods should be obvious).

Tested with VC 2005 and 2008. Does not build with VC 2010 currently.

The most interesting bit is probably in test_suffixarray, which implements the three suffix-array based string searchers previously described on this blog. See previous posts :

cbloom rants 06-17-10 - Suffix Array Neighboring Pair Match Lens
cbloom rants 09-23-11 - Morphing Matching Chain
cbloom rants 09-25-11 - More on LZ String Matching
cbloom rants 09-27-11 - String Match Stress Test
cbloom rants 09-28-11 - Algorithm - Next Index with Lower Value
cbloom rants 09-28-11 - String Matching with Suffix Arrays
cbloom rants 10-02-11 - How to walk binary interval tree
cbloom rants 09-24-11 - Suffix Tries 1
cbloom rants 09-24-11 - Suffix Tries 2
cbloom rants 09-26-11 - Tiny Suffix Note
cbloom rants 09-29-11 - Suffix Tries 3 - On Follows with Path Compression

cbloom rants 09-30-11 - String Match Results Part 1
cbloom rants 09-30-11 - String Match Results Part 2
cbloom rants 09-30-11 - String Match Results Part 2b
cbloom rants 09-30-11 - String Match Results Part 3
cbloom rants 09-30-11 - String Match Results Part 4
cbloom rants 09-30-11 - String Match Results Part 5 + Conclusion
cbloom rants 10-01-11 - String Match Results Part 6

StringMatchTest includes :


/*
 * divsufsort.c for libdivsufsort-lite
 * Copyright (c) 2003-2008 Yuta Mori All Rights Reserved.
 *

/* LzFind.c -- Match finder for LZ algorithms
2009-04-22 : Igor Pavlov : Public domain */

/*
    MMC (Morphing Match Chain)
    Match Finder
    Copyright (C) Yann Collet 2010-2011

StringMatchTest like all cbloom.com software is released under zlib license (basically free for all uses).

old rants