linux-kernel - Re: [PATCH] block: strip out locking optimization in put_io

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Message-ID: <CA+55aFzVmb_DfPPDyeON9X2KAii79g=NszPi+k9jrNLdQS7+4g@mail.gmail.com>
Date:	Thu, 9 Feb 2012 10:07:35 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Shaohua Li <shaohua.li@...el.com>, Jens Axboe <axboe@...nel.dk>,
	Vivek Goyal <vgoyal@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Knut Petersen <Knut_Petersen@...nline.de>, mroos@...ux.ee
Subject: Re: [PATCH] block: strip out locking optimization in put_io_context()

On Thu, Feb 9, 2012 at 9:59 AM, Tejun Heo <tj@...nel.org> wrote:
>
> * What exactly is the test and what do you measure?  What does "12%
>  regression" mean?  Is it wallclock time or CPU time?  If it's CPU
>  time, does systime increase dominate the regression?

Shaohua, it might be interesting to see a profile of the bad case.

Now, quite often these kinds of things don't show anything at all -
it's just due to cache issues and there's no obvious "we hold spinlock
X for 15 seconds total". But if it's actual lock contention rather
than just "more scheduling of worker threads", it should show up in
the profile quite clearly.

That said, I do think the RCU approach is the right one. The whole
delayed deallocation (and the replacement patch with rwlocks) really
smells like "badly done RCU-like behavior" to me.

                  Linus
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