[PATCH] readahead: add notes on readahead size
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From: Wu Fengguang on 25 Feb 2010 21:50 > 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(a)selenic.com> CC: Christian Ehrhardt <ehrhardt(a)linux.vnet.ibm.com> Signed-off-by: Wu Fengguang <fengguang.wu(a)intel.com> --- 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. + */ + #include <linux/kernel.h> #include <linux/fs.h> #include <linux/memcontrol.h> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |