From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: [PATCH 4/8] determine the existence of private futexes at the first thread creation
Date: Fri, 23 Jun 2017 13:05:35 -0400 [thread overview]
Message-ID: <20170623170535.GM1627@brightrain.aerifal.cx> (raw)
In-Reply-To: <f21ea165c38a4dfc9ac0b9a5b95f2a5622697213.1498228733.git.Jens.Gustedt@inria.fr>
On Tue, Jun 20, 2017 at 10:35:28PM +0200, Jens Gustedt wrote:
> The current strategie to deal with kernels that don't implement private
> futexes is to
>
> - test if the call works with FUTEX_PRIVATE
> - fallback to a call without if it doesn't
>
> This forces an overhead for both sides, Linux'es with and without private
> futexes. For those with, it adds a superflouous branch instruction to all
> calls. For those without, it add the whole call overhead of a syscall.
This was intentional, the idea being that a 100% predictable branch in
a path where a syscall is being made anyway is much less expensive
than a GOT address load that gets hoisted all the way to the top of
the function and affects even code paths that don't need to make the
syscall. Whether it was a choice that makes sense overall, I'm not
sure, but that was the intent.
Rich
next prev parent reply other threads:[~2017-06-23 17:05 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-23 14:38 [PATCH 0/8] the new __lock and follow up patches Jens Gustedt
2017-06-16 7:11 ` [PATCH 1/8] (V2) a new lock algorithm with lock value and CS counts in the same atomic int Jens Gustedt
2017-12-20 21:58 ` Rich Felker
2017-12-21 11:06 ` Jens Gustedt
2017-12-21 23:34 ` Rich Felker
2018-01-03 14:08 ` automated coding style Jens Gustedt
2018-01-11 4:41 ` Samuel Holland
2018-01-11 8:28 ` Jens Gustedt
2017-06-20 13:25 ` [PATCH 3/8] revise the definition of multiple basic locks in the code Jens Gustedt
2017-06-20 19:08 ` [PATCH 6/8] use the new lock algorithm for malloc Jens Gustedt
2017-06-23 15:01 ` Jens Gustedt
2017-06-20 19:41 ` [PATCH 2/8] consistently use the LOCK an UNLOCK macros Jens Gustedt
2017-06-20 19:44 ` [PATCH 5/8] separate the fast parts of __lock and __unlock into a .h file that may be used by other TU Jens Gustedt
2017-06-20 20:35 ` [PATCH 4/8] determine the existence of private futexes at the first thread creation Jens Gustedt
2017-06-23 17:05 ` Rich Felker [this message]
2017-06-23 17:16 ` Jens Gustedt
2017-06-23 21:48 ` Jens Gustedt
2017-06-23 22:08 ` Rich Felker
2017-06-23 23:42 ` Jens Gustedt
2017-06-24 0:53 ` Rich Felker
2017-06-24 8:00 ` Jens Gustedt
2017-06-24 10:12 ` flag 128 Jens Gustedt
2017-06-24 22:28 ` Rich Felker
2017-06-24 22:23 ` [PATCH 4/8] determine the existence of private futexes at the first thread creation Rich Felker
2017-06-22 21:17 ` [PATCH 7/8] implement __unlock_requeue Jens Gustedt
2017-06-22 21:42 ` [PATCH 8/8] implement the local lock for conditions with __lock & Co Jens Gustedt
2017-06-23 14:57 ` [PATCH 0/8] the new __lock and follow up patches Jens Gustedt
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