From: David Howells <dhowells@redhat.com>
To: Michel Lespinasse <walken@google.com>
Cc: dhowells@redhat.com, Andrew Morton <akpm@linux-foundation.org>,
Linux-MM <linux-mm@kvack.org>, Ying Han <yinghan@google.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: rwsem: down_read_unfair() proposal
Date: Wed, 05 May 2010 12:09:59 +0100 [thread overview]
Message-ID: <23977.1273057799@redhat.com> (raw)
In-Reply-To: <20100505104807.GB32643@google.com>
Michel Lespinasse <walken@google.com> wrote:
> I only said it was doable :) Not done with the implementation yet, but I can
> describe the general idea if that helps. The high part of the rwsem is
> decremented by two for each thread holding or trying to acquire a write
> lock;
That would mean you're reducing the capacity of the upper counter by one since
the high part must remain negative if we're to be able to check it for
non-zeroness by checking the sign flag. That means a maximum of 2^14-1 writers
queued on a 32-bit box (16384), but we can have more threads than that (up to
~32767).
Currently, we can have a maximum of 32767 writers+readers queued as we only
decrement the upper counter by 1 each time.
On a 64-bit box, the limitations go away for all practical purposes.
David
WARNING: multiple messages have this Message-ID (diff)
From: David Howells <dhowells@redhat.com>
To: Michel Lespinasse <walken@google.com>
Cc: dhowells@redhat.com, Andrew Morton <akpm@linux-foundation.org>,
Linux-MM <linux-mm@kvack.org>, Ying Han <yinghan@google.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: rwsem: down_read_unfair() proposal
Date: Wed, 05 May 2010 12:09:59 +0100 [thread overview]
Message-ID: <23977.1273057799@redhat.com> (raw)
In-Reply-To: <20100505104807.GB32643@google.com>
Michel Lespinasse <walken@google.com> wrote:
> I only said it was doable :) Not done with the implementation yet, but I can
> describe the general idea if that helps. The high part of the rwsem is
> decremented by two for each thread holding or trying to acquire a write
> lock;
That would mean you're reducing the capacity of the upper counter by one since
the high part must remain negative if we're to be able to check it for
non-zeroness by checking the sign flag. That means a maximum of 2^14-1 writers
queued on a 32-bit box (16384), but we can have more threads than that (up to
~32767).
Currently, we can have a maximum of 32767 writers+readers queued as we only
decrement the upper counter by 1 each time.
On a 64-bit box, the limitations go away for all practical purposes.
David
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next prev parent reply other threads:[~2010-05-05 11:10 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-05 3:20 rwsem: down_read_unfair() proposal Michel Lespinasse
2010-05-05 3:20 ` Michel Lespinasse
2010-05-05 10:03 ` David Howells
2010-05-05 10:03 ` David Howells
2010-05-05 10:36 ` Michel Lespinasse
2010-05-05 10:36 ` Michel Lespinasse
2010-05-06 23:26 ` Mike Waychison
2010-05-06 23:26 ` Mike Waychison
2010-05-05 10:06 ` David Howells
2010-05-05 10:06 ` David Howells
2010-05-05 10:48 ` Michel Lespinasse
2010-05-05 10:48 ` Michel Lespinasse
2010-05-05 11:09 ` David Howells [this message]
2010-05-05 11:09 ` David Howells
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