>>>>> "Chris" == Chris Mason <[email protected]> writes:
Chris> 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?
Chris> Ok, I did some more work to split out the two cases (block device inode
Chris> writeback and log flushing).
Chris> I patched jbd's log_do_checkpoint to put all the blocks it
Chris> wanted to write in a radix tree, then send them all down in
Chris> order at the end. The elevator should be helping here, but jbd
Chris> is sending down 2,000 to 3,000 blocks during the checkpoint and
Chris> upping nr_requests alone didn't seem to be doing the trick.
That seems like a really high number to me, in terms of the number of
blocks being check pointed here. Should we be more aggressively
writing journal blocks? Or having sub-transactions so we can write
smaller atomic chunks, while still not forcing them to disk?
I dunno, I'm probably smoking something here.
Just thinking out loud, what's the worst possible layout of inodes and
such that can be written in ext3? And can we generate a test case
which re-creates that layout and then the handling of it? Is it when
we try to write N inodes, and we do inode 1, then N, then 2, then N-1,
etc? So that the writes are spread out all over the place and don't
have any clustering in them at all?
Chris> Unpatched ext3 would break down into seeks after 8 kernel trees
Chris> are created (222MB each). With the radix sorting, the first 15
Chris> kernel trees are created quickly, and then we slow down.
Chris> So I waited until around the 25th kernel tree was created, hit
Chris> ctrl-c and ran sync. vmstat showed writes going at 2MB/s, and
Chris> sysrq-w showed sync was running the block device inode for most
Chris> of the 2MB/s period.
How much data was written overall at this point? And was it sorted at
all, or were just sub-chunks of it sorted?
Chris> It looks as though the dirty pages on the block device inode
Chris> are spread out far enough that we're not getting good streaming
Chris> writes. Mark Fasheh ran on a bigger raid array, where
Chris> performance was consistently good for the whole run. I'm
Chris> assuming the larger write cache on the array was able to group
Chris> the data writes with the metadata on disk, while my poor little
Chris> sata drive wasn't. Dave Chinner hinted that xfs is probably
Chris> suffering a similar problem, which is usually fixed by backing
Chris> the FS with stripes and big raid.
It sounds like Mark Fasheh just needs to have a bigger test case to
also hit the same wall. His hardware just moves it out a bit.
Chris> My vaporware FS is able to maintain speed through the run
Chris> because the allocator tries to keep data and metadata grouped
Chris> into 256mb chunks, and so they don't end up mingling on disk
Chris> until things get full.
So what happens when your vaporFS is told to allocate in 128Mb chunks
doing this same test? I assume it gets into the same type of problem?
I'm happy to see these tests happening, even if I don't have alot to
contribute otherwise. :[
One thought, do you have a list of the blocks being written for the
dirty block device inode? Should we be trying to push them out
whenenever we write data blocks which are near by?
I wonder how ext4 will do with this?
John
-
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/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
