From: Andrea Arcangeli <andrea@suse.de>
To: Andrew Morton <akpm@osdl.org>
Cc: nickpiggin@yahoo.com.au, riel@redhat.com,
marcelo.tosatti@cyclades.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH][5/?] count writeback pages in nr_scanned
Date: Thu, 6 Jan 2005 06:17:07 +0100 [thread overview]
Message-ID: <20050106051707.GP4597@dualathlon.random> (raw)
In-Reply-To: <20050105210539.19807337.akpm@osdl.org>
On Wed, Jan 05, 2005 at 09:05:39PM -0800, Andrew Morton wrote:
> Andrea Arcangeli <andrea@suse.de> wrote:
> >
> > The fix is very simple and it is to call wait_on_page_writeback on one
> > of the pages under writeback.
>
> eek, no. That was causing waits of five seconds or more. Fixing this
> caused the single greatest improvement in page allocator latency in early
> 2.5. We're totally at the mercy of the elevator algorithm this way.
>
> If we're to improve things in there we want to wait on _any_ eligible page
> becoming reclaimable, not on a particular page.
I told you one way to fix it. I didn't guarantee it was the most
efficient one.
I sure agree waiting on any page to complete writeback is going to fix
it too. Exactly because this page was a "random" page anyway.
Still my point is that this is a bug, and I prefer to be slow and safe
like 2.4, than fast and unreliable like 2.6.
The slight improvement you suggested of waiting on _any_ random
PG_writeback to go away (instead of one particular one as I did in 2.4)
is going to fix the write throttling equally too as well as the 2.4
logic, but without introducing slowdown that 2.4 had.
It's easy to demonstrate: exactly because the page we pick is random
anyway, we can pick the first random one that has seen PG_writeback
transitioning from 1 to 0. The guarantee we get is the same in terms of
safety of the write throttling, but we also guarantee the best possible
latency this way. And the HZ/x hacks to avoid deadlocks will magically
go away too.
next prev parent reply other threads:[~2005-01-06 5:16 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-01-03 17:25 [PATCH][5/?] count writeback pages in nr_scanned Rik van Riel
2005-01-05 10:08 ` Andrew Morton
2005-01-05 18:06 ` Andrea Arcangeli
2005-01-05 18:50 ` Rik van Riel
2005-01-05 17:49 ` Marcelo Tosatti
2005-01-05 21:44 ` Andrew Morton
2005-01-05 20:32 ` Marcelo Tosatti
2005-01-05 23:51 ` Nick Piggin
2005-01-06 1:27 ` Rik van Riel
2005-01-06 1:33 ` Nick Piggin
2005-01-06 1:37 ` Andrew Morton
2005-01-06 1:40 ` Nick Piggin
2005-01-06 1:52 ` Andrea Arcangeli
2005-01-06 1:36 ` Andrew Morton
2005-01-06 3:42 ` Rik van Riel
2005-01-06 3:50 ` Nick Piggin
2005-01-06 4:26 ` Andrew Morton
2005-01-06 4:35 ` Nick Piggin
2005-01-06 4:47 ` Andrew Morton
2005-01-06 4:55 ` Nick Piggin
2005-01-06 5:03 ` Andrea Arcangeli
2005-01-06 8:06 ` Jens Axboe
2005-01-06 8:16 ` memory barrier in ll_rw_blk.c (was Re: [PATCH][5/?] count writeback pages in nr_scanned) Nick Piggin
2005-01-06 8:32 ` Jens Axboe
2005-01-06 8:53 ` Nick Piggin
2005-01-06 12:00 ` Jens Axboe
2005-01-06 4:59 ` [PATCH][5/?] count writeback pages in nr_scanned Andrea Arcangeli
2005-01-06 5:05 ` Andrew Morton
2005-01-06 5:17 ` Andrea Arcangeli [this message]
2005-01-06 5:19 ` Nick Piggin
2005-01-06 5:25 ` Andrea Arcangeli
2005-01-06 5:36 ` Nick Piggin
2005-01-06 5:44 ` Nick Piggin
2005-01-06 5:37 ` Andrew Morton
2005-01-06 5:59 ` Andrea Arcangeli
2005-01-06 13:28 ` Rik van Riel
2005-01-06 5:32 ` Andrew Morton
2005-01-06 5:46 ` Andrea Arcangeli
2005-01-06 5:59 ` Andrew Morton
2005-01-06 6:16 ` Andrea Arcangeli
2005-01-06 5:06 ` Nick Piggin
2005-01-06 5:21 ` Andrea Arcangeli
2005-01-05 23:26 ` Andrew Morton
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