On Fri, Mar 19 2010, Shaohua Li wrote: > 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 4MB is definitely a huge read-ahead size, but I do wonder why it would perform that much worse than a 128KB window. If you narrow your test down to a single disk (or something simpler, at least), how does 4MB compare to 128KB? With 6GB of memory, you should not run into read-ahead memory thrashing. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
