From: Fujii Masao <masao.fujii@oss.nttdata.com>
To: Ants Aasma <ants@cybertec.at>
Cc: Julien Rouhaud <rjuju123@gmail.com>
Cc: Tharakan, Robins <tharar@amazon.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: track_planning causing performance regression
Date: Tue, 30 Jun 2020 22:40:09 +0900
Message-ID: <fdda5b0a-b433-b162-97ca-2093dacd40ec@oss.nttdata.com> (raw)
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<5725cc95-6aea-d3a8-6ea5-36acf780a55d@oss.nttdata.com>
<CANwKhkPUBcSBMwR1=dgGe-7cZriQ=Rge4j_xv5wHni+YM_CgSA@mail.gmail.com>
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
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To: pgsql-hackers@postgresql.org
Cc: masao.fujii@oss.nttdata.com, ants@cybertec.at, rjuju123@gmail.com, tharar@amazon.com
Subject: Re: track_planning causing performance regression
In-Reply-To: <fdda5b0a-b433-b162-97ca-2093dacd40ec@oss.nttdata.com>
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