On Fri, Mar 19, 2010 at 08:59:48AM +0800, Shaohua Li wrote: > On Thu, Mar 18, 2010 at 08:53:13PM +0800, Chris Mason wrote: > > On Thu, Mar 18, 2010 at 09:42:57AM +0800, Shaohua Li wrote: > > > Btrfs uses below equation to calculate ra_pages: > > > fs_info->bdi.ra_pages = max(fs_info->bdi.ra_pages, > > > 4 * 1024 * 1024 / PAGE_CACHE_SIZE); > > > is the max() a typo of min()? This makes the readahead size is 4M by > > > default, > > > which is too big. > > > > Looks like things have changed since I tuned that number. Fengguang has > > been busy ;) > > > > > I have a system with 16 CPU, 6G memory and 12 sata disks. I create a > > > btrfs for > > > each disk, so this isn't a raid setup. The test is fio, which has 12 > > > tasks to > > > access 12 files for each disk. The fio test is mmap sequential read. I > > > measure > > > the performance with different readahead size: > > > ra size io throughput > > > 4M 268288 k/s > > > 2M 367616 k/s > > > 1M 431104 k/s > > > 512K 474112 k/s > > > 256K 512000 k/s > > > 128K 538624 k/s > > > The 4M default readahead size has poor performance. > > > I also does sync sequential read test, the test difference in't that big. > > > But > > > the 4M case still has about 10% drop compared to the 512k case. > > > > I'm surprised the 4M is so much slower. At any rate, the larger size > > was selected because btrfs checksumming means we need a bigger buffer to > > keep the disks saturated. Were you on a fancy intel box with hardware > > crc32c enabled? > yes, this machine supports sse4.2 instruction. Let me check the result with > checksum > disabled. Sounds no big difference with checksum disabled. I format the disks and redo the test: 128k ra: 539648 k/s 4m ra: 285696 k/s
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