From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: [PATCH] readahead: add notes on readahead size Date: Fri, 26 Feb 2010 10:48:37 +0800 Message-ID: <20100226024837.GA22859@localhost> References: <20100224031001.026464755@intel.com> <20100224031054.307027163@intel.com> <4B869682.9010709@linux.vnet.ibm.com> <20100226022907.GA22226@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andrew Morton , Jens Axboe , Matt Mackall , Chris Mason , Peter Zijlstra , Clemens Ladisch , Olivier Galibert , Vivek Goyal , Nick Piggin , Linux Memory Management List , "linux-fsdevel@vger.kernel.org" , LKML , Rik van Riel To: Christian Ehrhardt Return-path: Content-Disposition: inline In-Reply-To: <20100226022907.GA22226@localhost> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org > 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 CC: Christian Ehrhardt Signed-off-by: Wu Fengguang --- 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 #include #include -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org