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Date: Fri, 26 Feb 2010 08:23:40 +0100
From: Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
To: Wu Fengguang <fengguang.wu@...el.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
Unfortunately without a chance to measure this atm, this patch now looks
really good to me.
Thanks for adapting it to a read-ahead only per mem limit.
Acked-by: Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
Wu Fengguang wrote:
> 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;
--
Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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