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.
Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
