On Fri, Feb 26, 2010 at 10:48:37AM +0800, Wu Fengguang wrote: > > 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. > > btw, I wrote some comments to summarize the now complex readahead size > rules.. > > == > readahead: add notes on readahead size > > Basically, currently the default max readahead size > - is 512k > - is boot time configurable with "readahead=" > and is auto scaled down: > - for small devices > - for small memory systems (read-around size alone) > > CC: Matt Mackall <mpm@xxxxxxxxxxx> > CC: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > --- > mm/readahead.c | 22 ++++++++++++++++++++++ > 1 file changed, 22 insertions(+) > > --- linux.orig/mm/readahead.c 2010-02-26 10:11:41.000000000 +0800 > +++ linux/mm/readahead.c 2010-02-26 10:11:55.000000000 +0800 > @@ -7,6 +7,28 @@ > * Initial version. > */ > > +/* > + * Notes on readahead size. > + * > + * The default max readahead size is VM_MAX_READAHEAD=512k, > + * which can be changed by user with boot time parameter "readahead=" > + * or runtime interface "/sys/devices/virtual/bdi/default/read_ahead_kb". > + * The latter normally only takes effect in future for hot added devices. > + * > + * The effective max readahead size for each block device can be accessed with > + * 1) the `blockdev` command > + * 2) /sys/block/sda/queue/read_ahead_kb > + * 3) /sys/devices/virtual/bdi/$(env stat -c '%t:%T' /dev/sda)/read_ahead_kb > + * > + * They are typically initialized with the global default size, however may be > + * auto scaled down for small devices in add_disk(). NFS, software RAID, btrfs > + * etc. have special rules to setup their default readahead size. > + * > + * The mmap read-around size typically equals with readahead size, with an > + * extra limit proportional to system memory size. For example, a 64MB box > + * will have a 64KB read-around size limit, 128MB mem => 128KB limit, etc. > + */ > + Great. I was confused among so many ways to control read ahead size. This documentation helps a lot. Vivek > #include <linux/kernel.h> > #include <linux/fs.h> > #include <linux/memcontrol.h> -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
