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Re: [PATCH][AArch64] Single thread lowlevellock optimization
- From: Torvald Riegel <triegel at redhat dot com>
- To: Szabolcs Nagy <szabolcs dot nagy at arm dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>, nd at arm dot com
- Date: Tue, 20 Jun 2017 15:47:45 +0200
- Subject: Re: [PATCH][AArch64] Single thread lowlevellock optimization
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On Fri, 2017-06-16 at 17:26 +0100, Szabolcs Nagy wrote:
> Do single thread lock optimization in aarch64 libc. Atomic operations
> hurt the performance of some single-threaded programs using stdio
> (usually getc/putc in a loop).
> Ideally such optimization should be done at a higher level and in a
> target independent way as in
> but that approach will need more discussion so do it in lowlevellocks,
> similarly to x86, until there is consensus.
I disagree that this is sufficient reason to do the right thing here
(ie, optimize in the high-level algorithm). What further discussion is
needed re the high-level use case?
> Differences compared to the current x86_64 behaviour:
> - The optimization is not silently applied to shared locks, in that
> case the build fails.
> - Unlock assumes the futex value is 0 or 1, there are no waiters to
> wake (that would not work in single thread and libc does not use
> such locks, to be sure lll_cond* is undefed).
> This speeds up a getchar loop about 2-4x depending on the cpu,
> while only cause around 5-10% regression for the multi-threaded case
What measurement of what benchmark resulted in that number (the latter
one)? Without details of what you are measuring this isn't meaningful.
> (other libc internal locks are not expected to be performance
> critical or significantly affected by this change).
Why do you think this is the case?