linux-kernel - Re: + lib-vsprintfc-even-faster-decimal-conversion.patch added to -mm tree

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Date:	Thu, 19 Mar 2015 15:17:10 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>,
	Denis Vlasenko <vda.linux@...glemail.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: + lib-vsprintfc-even-faster-decimal-conversion.patch added to -mm tree

On Wed, Mar 18, 2015 at 1:04 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Mon, 16 Mar 2015 18:19:41 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:
>
>> Rasmus, I redid benchmarks:
>
> tl;dr ;)  Is this an ack or a nack?

New code executes slower for some input on one CPU I've benchmarked,
both with -O2 and -Os (Core 2 Duo E6550). The slowdown is in 2-20% range
depending on exact number being converted and is reproducible.
With -O2 "bad" range is roughly 100-70000, with -Os it is 100-1000 and
around 100000.

On another CPU (more modern Core i5-something), new code is uniformly
faster giving advertised 10-20-35-40% speedups (yay!)

The ideal situation is still being
a) one system call to push PIDs into userspace in bulk
  (memcpy directly from pid_ns->pidmap[i]),
b) one system call to fetch data in binary to userspace given PID or
set of PIDs,

without all of this string and /proc overhead.

    Alexey
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