linux-kernel - Re: [PATCH] [15/58] i386: Rewrite sched

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20070720054740.GA13555@Krystal>
Date:	Fri, 20 Jul 2007 01:47:40 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andi Kleen <ak@...e.de>, patches@...-64.org,
	linux-kernel@...r.kernel.org, Daniel Walker <dwalker@...sta.com>
Subject: Re: [PATCH] [15/58] i386: Rewrite sched_clock (cmpxchg8b)

* Nick Piggin (nickpiggin@...oo.com.au) wrote:
> Mathieu Desnoyers wrote:
> 
> >I tried it with and without the LOCK prefix on my Pentium 4.
> >
> >Locked cmpxchg8b : 90 cycles
> >Non locked cmpxchg8b: 30 cycles
> >sti: 166 cycles
> >cli: 159 cycles
> >
> >So, hrm, even if we use the locked version, it is still much faster than
> >the sti/cli. I am thoughtful about the comment in asm-i386/system.h:
> 
> Curious: what does it look like if the memory is not in cache? I
> found that cmpxchg is relatively slower than other rmw instructions
> in that case.
> 

Actually, I have just seen that cmpxchg64 and cmpxchg64_local are
doing exactly this and they are already implemented in asm-i386/system.h.

A quick test: I am doing clflush in a loop (substracting its time from the
following loops) to have a memory hit when I do cmpxchg. This is the
result of just the cmpxchg8b:

non locked cmpxchg8b: 583.37 cycles
locked cmpxchg8b: 650.48 cycles
rmw in 3 operations: 581.43 cycles

So the locked cmpxchg is 67 cycles slower than the non locked cmpxchg,
which fits with my 30 vs 90 cycles. rmw is a tiny bit faster than
cmpxchg8b (2 cycles), but nothing to call home about.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/