[Mesa-dev] [PATCH] os: add spinlocks

[Thomas Hellstrom]

OK, Some info to back this up, please see inline.

On 12/15/2010 01:20 PM, Thomas Hellstrom wrote:
> On 12/15/2010 09:23 AM, Marek Olšák wrote:
>> On Tue, Dec 14, 2010 at 8:10 PM, Thomas Hellstrom 
>> <thellstrom at vmware.com <mailto:thellstrom at vmware.com>> wrote:
>>
>>     Hmm,
>>
>>     for the uninformed, where do we need to use spinlocks in gallium
>>     and how do
>>     we avoid using them on an UP system?
>>
>>
>> I plan to use spinlocks to guard very simple code like the macro 
>> remove_from_list, which might be, under some circumstances, called 
>> too often. Entering and leaving a mutex is quite visible in callgrind.
>>
>> What does UP stand for?
>>
>> Marek
>
> I've also noted that mutexes show up high on profiling on a number of 
> occasions, but
>
> Linux mutexes should theoretically be roughly as fast as spinlocks 
> unless there is high contention. (They shouldn't call into the kernel 
> unless there is contention). If this is not the case it is worth 
> figuring out why.

At a first glance, looking at this article,

http://www.alexonlinux.com/pthread-mutex-vs-pthread-spinlock

spinlocks should be superior for things like quick list manipulation, 
but If I try that code with the second thread disabled (thus avoiding 
lock contention), it turns out that in the uncontended case, spinlocks 
are only some 13% faster.

>
> The problem with spinlocks in user-space is that a context switch may 
> occur in a locked section and that will have other processes trying to 
> lock spinning until the process holding the lock is scheduled again.
>
> On uni-processor (UP) systems spinlocks are particularly bad, since if 
> there is contention on a lock, the process will always continue to 
> spin until a timeout context switch, and thus behave much worse than a 
> mutex  which will context switch immediately.
> (This is of course unless pthread spinlocks does some magic I'm not 
> aware of)


Trying the code with two threads on a uni-processor desktop system, 
spinlocks are some 63% slower than mutexes. This is with a kernel tick 
of 1000HZ. Probably spinlock stats  would be much worse with a slower tick.

>
> In kernel space the situation is different, since spinlocks block 
> preemption and contention can thus never occur on uniprocessor systems.
>
> There are of course situations where user-space spinlocks are faster 
> (high contention and low chance of context switch), but that really 
> means the user needs to know exactly what he's doing and the drawbacks 
> of using spinlocks.
>
> I don't really think we should add spinlocks to gallium without
>
> a) Figuring out why, in your case, mutexes behave so much worse than 
> spinlocks. Do you expect high lock contention?
> b) adding some kind of warning about the drawbacks of using them.


Finally, trying to repeat the author's findings on my dual-processor 
system, Spinlocks are faster (but not nearly as much as the author 
states, but that's probably because my CPUs are faster hence lower lock 
contention), but the cpu-usage is roughly the same as in the mutex case.

Given this, I would advise strongly against building spinlocks into any 
code that might be run on a uni-processor system.  Particularly gallium 
utility code.
If we want to get rid of unnecessary locking overhead we should probably 
fix the code up to avoid taking the locks when not strictly needed.

/Thomas

>
> /Thomas
>

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