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Message-ID: <CAK1hOcOHNCjPmqE1uq=OCbpckXz-xFJx_gcQ4Q-TKHgKGVEP-Q@mail.gmail.com>
Date: Tue, 10 Feb 2015 21:52:54 +0100
From: Denys Vlasenko <vda.linux@...glemail.com>
To: musl <musl@...ts.openwall.com>
Subject: Re: Re: [PATCH] x86_64/memset: simple optimizations
On Tue, Feb 10, 2015 at 9:43 PM, Rich Felker <dalias@...ifal.cx> wrote:
> On Tue, Feb 10, 2015 at 09:27:17PM +0100, Denys Vlasenko wrote:
>> On Sat, Feb 7, 2015 at 2:06 PM, Rich Felker <dalias@...ifal.cx> wrote:
>> /* libc has incredibly messy way of doing this,
>> * typically requiring -lrt. We just skip all this mess */
>> static void get_mono(struct timespec *ts)
>> {
>> syscall(__NR_clock_gettime, CLOCK_MONOTONIC, ts);
>> }
>
> FWIW, this is a bad idea; you get syscall overhead in your
> measurements. If you just use clock_gettime (the function) you'll get
> vdso results (no syscall).
I repeat memset 32 times between reading timespamp.
Thus, even with "small" 20kb memset test
there are 640kb of writes to L1. This is bit enough
to make overhead insignificant.
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