Subject: Re: track_planning causing performance regression
To: Ants Aasma <ants@cybertec.at>
Cc: Julien Rouhaud <rjuju123@gmail.com>, "Tharakan, Robins"
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From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <fdda5b0a-b433-b162-97ca-2093dacd40ec@oss.nttdata.com>
Date: Tue, 30 Jun 2020 22:40:09 +0900
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On 2020/06/30 20:30, Ants Aasma wrote:
> On Tue, 30 Jun 2020 at 08:43, Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>> wrote:
> 
>      > The problem looks to be that spinlocks are terrible with overloaded CPU and a contended spinlock. A process holding the spinlock might easily get scheduled out leading to excessive spinning by everybody. I think a simple thing to try would be to replace the spinlock with LWLock.
> 
>     Yes. Attached is the POC patch that replaces per-counter spinlock with LWLock.
> 
> 
> Great. I think this is the one that should get considered for testing.
> 
>      > I did a prototype patch that replaces spinlocks with futexes, but was not able to find a workload where it mattered.
> 
>     I'm not familiar with futex, but could you tell me why you used futex instead
>     of LWLock that we already have? Is futex portable?
> 
> 
> Futex is a Linux kernel call that allows to build a lock that has uncontended cases work fully in user space almost exactly like a spinlock, while falling back to syscalls that wait for wakeup in case of contention. It's not portable, but probably something similar could be implemented for other operating systems. I did not pursue this further because it became apparent that every performance critical spinlock had already been removed.
> 
> To be clear, I am not advocating for this patch to get included. I just had the patch immediately available and it could have confirmed that using a better lock fixes things.

Understood. Thanks for the explanation!

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION