linux-kernel - Re: [PATCH 05/15] readahead: limit readahead size for small memory systems

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Date:	Fri, 26 Feb 2010 10:29:07 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Matt Mackall <mpm@...enic.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Clemens Ladisch <clemens@...isch.de>,
	Olivier Galibert <galibert@...ox.com>,
	Vivek Goyal <vgoyal@...hat.com>, Nick Piggin <npiggin@...e.de>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 05/15] readahead: limit readahead size for small memory
	systems

On Thu, Feb 25, 2010 at 11:25:54PM +0800, Christian Ehrhardt wrote:
> 
> 
> Wu Fengguang wrote:
>  > When lifting the default readahead size from 128KB to 512KB,
>  > make sure it won't add memory pressure to small memory systems.
>  >
>  > For read-ahead, the memory pressure is mainly readahead buffers consumed
>  > by too many concurrent streams. The context readahead can adapt
>  > readahead size to thrashing threshold well.  So in principle we don't
>  > need to adapt the default _max_ read-ahead size to memory pressure.
>  >
>  > For read-around, the memory pressure is mainly read-around misses on
>  > executables/libraries. Which could be reduced by scaling down
>  > read-around size on fast "reclaim passes".
>  >
>  > This patch presents a straightforward solution: to limit default
>  > readahead size proportional to available system memory, ie.
>  >                 512MB mem => 512KB readahead size
>  >                 128MB mem => 128KB readahead size
>  >                  32MB mem =>  32KB readahead size (minimal)
>  >
>  > Strictly speaking, only read-around size has to be limited.  However we
>  > don't bother to seperate read-around size from read-ahead size for now.
>  >
>  > CC: Matt Mackall <mpm@...enic.com>
>  > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> 
> What I state here is for read ahead in a "multi iozone sequential" 
> setup, I can't speak for real "read around" workloads.
> So probably your table is fine to cover read-around+read-ahead in one 
> number.

OK.

> I have tested 256MB mem systems with 512kb readahead quite a lot.
> On those 512kb is still by far superior to smaller readaheads and I 
> didn't see major trashing or memory pressure impact.

In fact I'd expect a 64MB box to also benefit from 512kb readahead :)

> Therefore I would recommend a table like:
>                 >=256MB mem => 512KB readahead size
>                   128MB mem => 128KB readahead size
>                    32MB mem =>  32KB readahead size (minimal)

So, I'm fed up with compromising the read-ahead size with read-around
size.

There is no good to introduce a read-around size to confuse the user
though.  Instead, I'll introduce a read-around size limit _on top of_
the readahead size. This will allow power users to adjust
read-ahead/read-around size at the same time, while saving the low end
from unnecessary memory pressure :) I made the assumption that low end
users have no need to request a large read-around size.

Thanks,
Fengguang
---
readahead: limit read-ahead size for small memory systems

When lifting the default readahead size from 128KB to 512KB,
make sure it won't add memory pressure to small memory systems.

For read-ahead, the memory pressure is mainly readahead buffers consumed
by too many concurrent streams. The context readahead can adapt
readahead size to thrashing threshold well.  So in principle we don't
need to adapt the default _max_ read-ahead size to memory pressure.

For read-around, the memory pressure is mainly read-around misses on
executables/libraries. Which could be reduced by scaling down
read-around size on fast "reclaim passes".

This patch presents a straightforward solution: to limit default
read-ahead size proportional to available system memory, ie.
                512MB mem => 512KB readahead size
                128MB mem => 128KB readahead size
                 32MB mem =>  32KB readahead size

CC: Matt Mackall <mpm@...enic.com>
CC: Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---
 mm/filemap.c   |    2 +-
 mm/readahead.c |   22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

--- linux.orig/mm/filemap.c	2010-02-26 10:04:28.000000000 +0800
+++ linux/mm/filemap.c	2010-02-26 10:08:33.000000000 +0800
@@ -1431,7 +1431,7 @@ static void do_sync_mmap_readahead(struc
 	/*
 	 * mmap read-around
 	 */
-	ra_pages = max_sane_readahead(ra->ra_pages);
+	ra_pages = min(ra->ra_pages, roundup_pow_of_two(totalram_pages / 1024));
 	if (ra_pages) {
 		ra->start = max_t(long, 0, offset - ra_pages/2);
 		ra->size = ra_pages;
--
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