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Re: [PATCH][AArch64] Single thread lowlevellock optimization

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?

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