Re: [PATCH] NUMA spinlock [BZ #23962]

[kemi <kemi.wang@intel.com>]

On 2019/1/15 下午8:36, Torvald Riegel wrote:
> On Tue, 2019-01-15 at 10:28 +0800, kemi wrote:
>>>> "Scalable spinlock" is something of an oxymoron.
>>>
>>> No, that's not true at all.  Most high-performance shared-memory
>>> synchronization constructs (on typical HW we have today) will do some kind
>>> of spinning (and back-off), and there's nothing wrong about it.  This can
>>> scale very well. 
>>>
>>>> Spinlocks are for
>>>> situations where contention is extremely rare,
>>>
>>> No, the question is rather whether the program needs blocking through the
>>> OS (for performance, or for semantics such as PI) or not.  Energy may be
>>> another factor.  For example, glibc's current mutexes don't scale well on
>>> short critical because there's not enough spinning being done.
>>>
>>
>> yes. That's why we need pthread.mutex.spin_count tunable interface before.
> 
> I don't think we need the tunable interface before that.  Where we need to
> improve performance most is for applications that don't want to bother
> tuning their mutexes -- that's where the broadest gains are overall, I
> think.
> 
> In turn, that means that we have spinning and back-off that give good
> average-case performance -- whether that's through automatic tuning of
> those two things at runtime, or through static default values that we do
> regular performance checks for in the glibc community. 
> 

Spinning and proportional back-off with auto tuning has been proposed for several years
and never got it merged in upstream kernel.
IMHO, this is because MCS-style lock wins that battle.

Could you tell me why we should consider backoff rather than MCS-style lock?

> From that perspective, the tunable interface is a nice addition that can
> allow users to fine-tune the setting, but it's not how users would enable
> it.
> 
>> But, that's not enough. When tunable is not the bottleneck, the simple busy-waiting
>> algorithm of current adaptive mutex is the major negative factor which degrades mutex
>> performance.
> 
> Note that I'm not advocating for focusing on just the adaptive mutex type. 
> IMO, adding this type was a mistake because whether to spin or not does not
> affect semantics of the mutexes.  Performance hints shouldn't be done via a
> mutex' type, and all mutex implementations should consider to spin at least
> a little.
> 
> If we just do something about the adaptive mutexes, then I guess this will
> reach few users.  I believe most applications just don't use them, and the
> current implementation of adaptive mutexes is so simplistic that there's
> not much performance to be had by changing to adaptive mutexes (which is
> another reason for it having few users).
> 

Generally, I agree with you.
May we tune adaptive mutex before applying these optimization to normal mutex.

>> That's why I proposed to use MCS-based spinning-waiting algorithm for adaptive
>> mutex.
> 
> MCS-style spinning (ie, spinning on memory local to the spinning thread) is
> helpful, but I think we should tackle spinning on global memory first (ie,
> on a location in the mutex, which is shared by all the threads trying to
> acquire it).  Of course, always including back-off.
> 
>> https://sourceware.org/ml/libc-alpha/2019-01/msg00279.html
>>
>> Also, if with very small critical section in the worklad, this new type of mutex 
>> with GNU extension PTHREAD_MUTEX_QUEUESPINNER_NP acts like MCS-spinlock, and performs
>> much better than original spinlock.
> 
> I don't think we want to have a new type for that.  It maybe useful for
> experimenting with it, but it shouldn't be exposed to users as a stable
> interface.
> 

I don't like to add a new type either.
As I said in the commit log, that's a trade-off to avoid ABI changed.
I am very glad to see that MCS-style lock can be used gracefully without
introducing a new type.

> Also, have you experimented with different kinds/settings of exponential
> back-off?  I just saw normal spinning in your implementation, no varying
> amounts of back-off.  The performance comparison should include back-off
> though, as that's one way to work around the contention problems (with a
> bigger hammer than local spinning of course, but can be effective
> nonetheless, and faster in low-contention cases).
> 

I didn't try back-off, because we don't have to include it if MCS-style lock is used.

> My guess is that a mix of local spinning on memory shared by a few threads
> running on cores that are close to each other would perform best (eg,
> similar to what's done in flat combining). 
> 
>> So, in some day, if adaptive mutex is tuned good enough, it should act like
>> mcs-spinlock (or NUMA spinlock) if workload has small critical section, and
>> performs like normal mutex if the critical section is too big to spinning-wait.
> 
> I agree in some way, but I think that the adaptive mutex type should just
> be an alias of the normal mutex type (for API compatibility reasons only). 
> And there could be other reasons than just critical-section-size that
> determine whether a thread should block using futexes or not.
> 

Agree.
I am justing moving toward that step by step.