On Tue, 22 May 2007 12:35:11 -0400 Chris Mason <[EMAIL PROTECTED]> wrote:
> On Wed, May 16, 2007 at 01:37:26PM -0700, Andrew Morton wrote: > > On Wed, 16 May 2007 16:14:14 -0400 > > Chris Mason <[EMAIL PROTECTED]> wrote: > > > > > On Wed, May 16, 2007 at 01:04:13PM -0700, Andrew Morton wrote: > > > > > The good news is that if you let it run long enough, the times > > > > > stabilize. The bad news is: > > > > > > > > > > create dir kernel-86 222MB in 15.85 seconds (14.03 MB/s) > > > > > create dir kernel-87 222MB in 28.67 seconds (7.76 MB/s) > > > > > create dir kernel-88 222MB in 18.12 seconds (12.27 MB/s) > > > > > create dir kernel-89 222MB in 19.77 seconds (11.25 MB/s) > > > > > > > > well hang on. Doesn't this just mean that the first few runs were > > > > writing > > > > into pagecache and the later ones were blocking due to dirty-memory > > > > limits? > > > > > > > > Or do you have a sync in there? > > > > > > > There's no sync, but if you watch vmstat you can clearly see the log > > > flushes, even when the overall create times are 11MB/s. vmstat goes > > > 30MB/s -> 4MB/s or less, then back up to 30MB/s. > > > > How do you know that it is a log flush rather than, say, pdflush > > hitting the blockdev inode and doing a big seeky write? > > Ok, I did some more work to split out the two cases (block device inode > writeback and log flushing). > > I patched jbd's log_do_checkpoint to put all the blocks it wanted to > write in a radix tree, then send them all down in order at the end. Side note: we already have all of that capability in the kernel: sync_inode(blockdev_inode, wbc) will do an ascending-LBA write of the whole blockdev. It could be that as a quick diddle, running sync_inode() in do-block-on-queue-congestion mode prior to doing the checkpoint would have some benefit. > The elevator should be helping here, but jbd is sending down 2,000 > to 3,000 blocks during the checkpoint and upping nr_requests alone > didn't seem to be doing the trick. > > Unpatched ext3 would break down into seeks after 8 kernel trees are > created (222MB each). With the radix sorting, the first 15 kernel trees > are created quickly, and then we slow down. > > So I waited until around the 25th kernel tree was created, hit ctrl-c > and ran sync. vmstat showed writes going at 2MB/s, and sysrq-w showed > sync was running the block device inode for most of the 2MB/s period. > > It looks as though the dirty pages on the block device inode are spread > out far enough that we're not getting good streaming writes. Mark > Fasheh ran on a bigger raid array, where performance was consistently > good for the whole run. I'm assuming the larger write cache on the > array was able to group the data writes with the metadata on disk, while > my poor little sata drive wasn't. Dave Chinner hinted that xfs is > probably suffering a similar problem, which is usually fixed by backing > the FS with stripes and big raid. > > My vaporware FS is able to maintain speed through the run because the > allocator tries to keep data and metadata grouped into 256mb chunks, > and so they don't end up mingling on disk until things get full. > > At any rate, it may be worth putzing with the writeback routines to try > and find dirty pages close by in the block dev inode when doing data > writeback. My guess is that ext3 should be going 1.5x to 2x faster for > this particular run, but that's a huge amount of complexity added so I'm > not convinced it is a great idea. Yes, this is a distinct disadvantage of the whole per-address-space writeback scheme - we're leaving IO scheduling optimisations on the floor, especially wrt the blockdev inode, but probably also wrt regular-file versus regular-file. Even if one makes the request queue tremendously huge, that won't help if there's dirty data close-by the disk head which hasn't even been put into the queue yet. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
