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From: Denys Vlasenko <vda.linux@googlemail.com>
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Subject: Re: Re: [PATCH] x86_64/memset: simple optimizations
Date: Tue, 10 Feb 2015 21:52:54 +0100
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On Tue, Feb 10, 2015 at 9:43 PM, Rich Felker <dalias@aerifal.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@aerifal.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.