Re: [PATCH v2] Single threaded stdio optimization

[Carlos O'Donell <carlos at redhat dot com>]

On 06/30/2017 08:15 AM, Szabolcs Nagy wrote:
> i didn't dig into the root cause of the regression (or
> why is static linking slower?), i would not be too
> worried about it since the common case for hot stdio
> loops is in single thread processes where even on x86
> the patch gives >2x speedup.

Regardless of the cause, the 15% regression on x86 MT performance
is serious, and I see no reason to push this into glibc 2.26. 
We can add it any time in 2.27, or the distros can pick it up with
a backport.

I would like to see a better characterization of the regression before
accepting this patch.

While I agree that common case for hot stdio loops is non-MT, there
are still MT cases, and 15% is a large double-digit loss.

Have you looked at the assembly differences? What is the compiler
doing differently?

When our a user asks "Why is my MT stdio 15% slower?" We owe them an
answer that is clear and concise.

-- 
Cheers,
Carlos.