From: Linus Torvalds <torvalds@osdl.org>
To: Nick Piggin <npiggin@suse.de>
Cc: Linux Memory Management <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Hugh Dickins <hugh@veritas.com>, Andrew Morton <akpm@osdl.org>,
Andrea Arcangeli <andrea@suse.de>,
David Miller <davem@davemloft.net>
Subject: Re: [patch 0/4] mm: de-skew page refcount
Date: Wed, 18 Jan 2006 08:38:44 -0800 (PST) [thread overview]
Message-ID: <Pine.LNX.4.64.0601180830520.3240@g5.osdl.org> (raw)
In-Reply-To: <20060118024106.10241.69438.sendpatchset@linux.site>
On Wed, 18 Jan 2006, Nick Piggin wrote:
>
> The following patchset (against 2.6.16-rc1 + migrate race fixes) uses the new
> atomic ops to do away with the offset page refcounting, and simplify the race
> that it was designed to cover.
>
> This allows some nice optimisations
Why?
The real downside is that "atomic_inc_nonzero()" is a lot more expensive
than checking for zero on x86 (and x86-64).
The reason it's offset is that on architectures that automatically test
the _result_ of an atomic op (ie x86[-64]), it's easy to see when
something _becomes_ negative or _becomes_ zero, and that's what
atomic_add_negative
atomic_inc_and_test
are optimized for (there's also "atomic_dec_and_test()" which reacts on
the count becoming zero, but that doesn't have a pairing: there's no way
to react to the count becoming one for the increment operation, so the
"atomic_dec_and_test()" is used for things where zero means "free it").
Nothing else can be done that fast on x86. Everything else requires an
insane "load, update, cmpxchg" sequence.
So I disagree with this patch series. It has real downsides. There's a
reason we have the offset.
I suspect that whatever "nice optimizations" you have are quite doable
without doing this count pessimization.
Linus
next prev parent reply other threads:[~2006-01-18 16:38 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-01-18 10:40 [patch 0/4] mm: de-skew page refcount Nick Piggin
2006-01-18 10:40 ` [patch 1/4] mm: page refcount use atomic primitives Nick Piggin
2006-01-18 10:40 ` [patch 2/4] mm: PageLRU no testset Nick Piggin
2006-01-19 17:48 ` Nikita Danilov
2006-01-19 18:10 ` Nick Piggin
2006-01-18 10:40 ` [patch 3/3] mm: PageActive " Nick Piggin
2006-01-18 14:13 ` Marcelo Tosatti
2006-01-19 14:50 ` Nick Piggin
2006-01-19 16:52 ` Marcelo Tosatti
2006-01-19 20:02 ` Nick Piggin
2006-01-19 21:41 ` Marcelo Tosatti
2006-01-18 10:41 ` [patch 4/4] mm: less atomic ops Nick Piggin
2006-01-18 16:38 ` Linus Torvalds [this message]
2006-01-18 17:05 ` [patch 0/4] mm: de-skew page refcount Nick Piggin
2006-01-18 19:27 ` Linus Torvalds
2006-01-19 14:00 ` Nick Piggin
2006-01-19 16:36 ` Linus Torvalds
2006-01-19 17:06 ` Nick Piggin
2006-01-19 17:27 ` Linus Torvalds
2006-01-19 17:38 ` Nick Piggin
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