From: Hugh Dickins <hughd@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Rik van Riel <riel@redhat.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 2/3] tmpfs radix_tree: locate_item to speed up swapoff
Date: Thu, 28 Jul 2011 02:26:38 -0700 (PDT) [thread overview]
Message-ID: <alpine.LSU.2.00.1107280159480.4995@sister.anvils> (raw)
In-Reply-To: <alpine.LSU.2.00.1107271801450.9888@sister.anvils>
On Wed, 27 Jul 2011, Hugh Dickins wrote:
> On Wed, 27 Jul 2011, Andrew Morton wrote:
> > On Tue, 19 Jul 2011 15:54:23 -0700 (PDT)
> > Hugh Dickins <hughd@google.com> wrote:
> >
> > > But it was a shock to find swapoff of a 500MB file 20 times slower
> > > on my laptop, taking 10 minutes; and at that rate it significantly
> > > slows down my testing.
> >
> > So it used to take half a minute? That was already awful.
> > Why? Was it IO-bound? It doesn't sound like it.
>
> No, not IO-bound at all.
I oversimplified: about 10 seconds of that was waiting for IO,
the rest (of 10 minutes or of half a minute) was cpu. It's the cpu
part of it which the change of radix tree has affected, for the worse.
> > How much did that 10 minutes improve?
>
> To 1 minute: still twice as slow as before. I believe that's because of
> the smaller nodes and greater height of the generic radix tree. I ought
> to experiment with a bigger RADIX_TREE_MAP_SHIFT to verify that belief
> (though I don't think tmpfs swapoff would justify raising it): will do.
Yes, raising RADIX_TREE_MAP_SHIFT from 6 to 10 (so on 32-bit the rnode
is just over 4kB, comparable with the old shmem's use of pages for this)
brings the time down considerably: still slower than before, but 12%
slower instead of twice as slow (or 20% slower instead of 3 times as
slow when comparing sys times).
Not that making a radix_tree_node need order:1 page would be sensible.
Hugh
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next prev parent reply other threads:[~2011-07-28 9:27 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-19 22:51 [PATCH 0/3] mm: tmpfs radix_tree swap leftovers Hugh Dickins
2011-07-19 22:52 ` [PATCH 1/3] radix_tree: clean away saw_unset_tag leftovers Hugh Dickins
2011-07-19 22:54 ` [PATCH 2/3] tmpfs radix_tree: locate_item to speed up swapoff Hugh Dickins
2011-07-27 23:28 ` Andrew Morton
2011-07-28 1:54 ` Hugh Dickins
2011-07-28 9:26 ` Hugh Dickins [this message]
2011-07-19 22:56 ` [PATCH 3/3] mm: clarify the radix_tree exceptional cases Hugh Dickins
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