Messages in this thread | | | Date | Fri, 26 Feb 2010 08:23:40 +0100 | From | Christian Ehrhardt <> | Subject | Re: [PATCH 05/15] readahead: limit readahead size for small memory systems |
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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@linux.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@selenic.com> >> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.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@selenic.com> > CC: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.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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Grüsse / regards, Christian Ehrhardt IBM Linux Technology Center, System z Linux Performance -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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