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Re: [Qemu-devel] [PATCH v3 4/5] qtest: precompute hex nibs

From: John Snow
Subject: Re: [Qemu-devel] [PATCH v3 4/5] qtest: precompute hex nibs
Date: Fri, 08 May 2015 12:22:34 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 05/08/2015 02:25 AM, Markus Armbruster wrote:
> Eric Blake <address@hidden> writes:
>> On 05/06/2015 10:18 AM, John Snow wrote:
>>>> To find out, add just buffering.  Something like this in your patch
>>>> instead of byte2hex():
>>>>          for (i = 0; i < len; i++) {
>>>> -            qtest_sendf(chr, "%02x", data[i]);
>>>> +            snprintf(&enc[i * 2], 2, "%02x", data[i]);
>>>>          }
>>>> If the speedup is pretty much entirely due to buffering (which I
>>>> suspect), then your commit message could use a bit of love :)
>>> When you're right, you're right. The difference may not be statistically
>>> meaningful, but with today's current planetary alignment, using
>>> sprintf() to batch the sends instead of my home-rolled nib computation
>>> function, I can eke out a few more tenths of a second.
>> I'm a bit surprised - making a function call per byte generally executes
>> more instructions than open-coding the conversion (albeit the branch
>> prediction in the hardware probably does fairly well over long strings,
>> since it is a tight and predictable loop).  Remember, sprintf() has to
>> decode the format string on every call (unless the compiler is smart
>> enough to open-code what sprintf would do).
> John's measurements show that the speed difference between snprintf()
> and a local copy of formatting code gets thoroughly drowned in noise.
> The snprintf() version takes 18 lines less, according to diffstat.  Less
> code, same measured performance, what's not to like?
> However, if you feel strongly about avoiding snprintf() here, I won't
> argue further.  Except for the commit message: it needs to be fixed not
> to claim avoiding "printf and friends" makes a speed difference.

My reasoning was the same as Markus's: the difference was so negligible
that I went with the "less home-rolled code" version.

I already staged this series without the nib functions and submitted the
snprintf version as its own patch with a less disparaging (to printf and
friends) commit message.

Any further micro-optimization is a waste of time to properly benchmark
and split hairs. I already dropped the test from ~14s to ~4s. Good enough.


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