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Message-ID: <20170618213209.260ae8e4@inria.fr>
Date: Sun, 18 Jun 2017 21:32:09 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] a new lock algorithm with lock value and CS
counts in the same atomic int
Hello Rich,
On Sun, 18 Jun 2017 12:04:59 -0400 Rich Felker <dalias@...c.org> wrote:
> > > Is there a reason __wait doesn't work?
> >
> > __wait doesn't fit here at all, for instance it manipulates a
> > separate int with waiters count.
>
> It accepts a null pointer for the waiters count, so that it can be
> used in contexts with or without one. Any cost of the additional
> conditional branches is dominated by syscall time.
Looking into it, I don't agree with you, Rich. Even with waiters set
to 0 it would to 100 spins before going into the syscall. This is much
of a waste, here, because we are just comming out of a spin (at the
first iteration) or we did spin around as long as the value was
positive.
I don't see why the cost of 100 spins would be dominated by the
syscall. If I remember correctly, the benchmarks that I made showed
about 10 memory operations for an unsuccessful syscall. This is why
the magic number for the initial spin is set to 10.
It might be benefitial to do a_spin for a while, if we know that CS
that are protected by this lock are really short, just some
cycles. But 100 is a far too big number, and in my benchmarks I found
not much indication of a benefit for it.
If we want code sharing with the rest of musl (which we should) I like
Alexander's idea of a __futexwait inline function much better.
Thanks
Jens
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