Re: [PATCH 05/15] readahead: limit readahead size for small memorysystems

[Christian Ehrhardt]

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@xxxxxxxxxxxxxxxxxx>


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@xxxxxxxxxxx>
> >  > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> >
> > 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@xxxxxxxxxxx>
> CC: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
>  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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