From: "Christopher Friesen" <cfriesen@nortel.com>
To: Esben Nielsen <simlo@phys.au.dk>
Cc: Arjan van de Ven <arjan@infradead.org>,
Daniel Walker <dwalker@mvista.com>, Ingo Molnar <mingo@elte.hu>,
linux-kernel@vger.kernel.org, Ulrich Drepper <drepper@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Andrew Morton <akpm@osdl.org>
Subject: Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace?
Date: Thu, 16 Feb 2006 14:23:48 -0600 [thread overview]
Message-ID: <43F4DF54.6030303@nortel.com> (raw)
In-Reply-To: <Pine.OSF.4.05.10602162057040.20911-100000@da410>
Esben Nielsen wrote:
> On Thu, 16 Feb 2006, Arjan van de Ven wrote:
>
>
>>On Thu, 2006-02-16 at 20:06 +0100, Esben Nielsen wrote:
>>>Why does the list have to be in userspace?
>>
>>because it's faster ;)
> Faster???
> As I see it, extra manipulations have to be done even in the non-congested
> case: Every time the lock is taken the locking thread has to add the lock
> to a the list, and reversely remove the lock from the list. I.e.
> instructions are _added_ to the fast path where you stay purely in
> userspace.
>
> I am ofcourse comparing to a solution where you do a syscall on everytime
> you do a lock.
The whole *point* of futexes is that on uncontested operations you don't
have to do a syscall. Thus, if you can avoid taking a syscall while
still getting reliability, you'll be faster.
Dropping to kernelspace isn't free.
Chris
next prev parent reply other threads:[~2006-02-16 20:24 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-02-16 9:41 [patch 0/6] lightweight robust futexes: -V3 Ingo Molnar
2006-02-16 16:33 ` Daniel Walker
2006-02-16 17:24 ` Ingo Molnar
2006-02-16 17:34 ` Daniel Walker
2006-02-16 19:06 ` [patch 0/6] lightweight robust futexes: -V3 - Why in userspace? Esben Nielsen
2006-02-16 19:34 ` Arjan van de Ven
2006-02-16 20:04 ` Esben Nielsen
2006-02-16 20:17 ` Esben Nielsen
2006-02-16 20:23 ` Christopher Friesen [this message]
2006-02-16 20:36 ` Ingo Molnar
2006-02-16 22:32 ` Esben Nielsen
2006-02-16 22:36 ` Ingo Molnar
2006-02-16 23:20 ` Esben Nielsen
2006-02-16 23:39 ` Ingo Molnar
2006-02-17 0:20 ` Esben Nielsen
2006-02-17 0:42 ` Ingo Molnar
2006-02-17 23:47 ` Andrew James Wade
2006-02-16 20:23 ` [patch 0/6] lightweight robust futexes: -V3 Ingo Molnar
2006-02-16 20:54 ` Daniel Walker
2006-02-16 21:26 ` Ingo Molnar
2006-02-16 21:50 ` Christopher Friesen
2006-02-16 21:55 ` Ingo Molnar
2006-02-16 20:47 ` Paul Jackson
2006-02-16 21:35 ` Ingo Molnar
2006-02-16 21:23 ` Paul Jackson
2006-02-16 21:50 ` Ingo Molnar
2006-02-17 4:56 ` Paul Jackson
2006-02-17 9:41 ` Ingo Molnar
2006-02-17 11:59 ` Ingo Molnar
2006-02-17 20:50 ` Paul Jackson
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