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Message-ID: <20130422143846.GA2675@suse.de>
Date: Mon, 22 Apr 2013 15:38:46 +0100
From: Mel Gorman <mgorman@...e.de>
To: Theodore Ts'o <tytso@....edu>
Cc: Jeff Moyer <jmoyer@...hat.com>, Dave Chinner <david@...morbit.com>,
Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
LKML <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>
Subject: Re: Excessive stall times on ext4 in 3.9-rc2
(Adding Jeff Moyer to the cc as I'm told he is interested in the blktrace)
On Fri, Apr 12, 2013 at 11:19:52AM -0400, Theodore Ts'o wrote:
> On Fri, Apr 12, 2013 at 02:50:42PM +1000, Dave Chinner wrote:
> > > If that is the case, one possible solution that comes to mind would be
> > > to mark buffer_heads that contain metadata with a flag, so that the
> > > flusher thread can write them back at the same priority as reads.
> >
> > Ext4 is already using REQ_META for this purpose.
>
> We're using REQ_META | REQ_PRIO for reads, not writes.
>
> > I'm surprised that no-one has suggested "change the IO elevator"
> > yet.....
>
> Well, testing to see if the stalls go away with the noop schedule is a
> good thing to try just to validate the theory.
>
I still haven't tested with a different elevator. While this bug is
relatively high priority for me, there are still are other issues in the way.
TLDR: Flusher writes pages very quickly after processes dirty a buffer. Reads
starve flusher writes.
Now the ugliness and being a windbag.
I collected blktrace and some other logs and they are available at
http://www.csn.ul.ie/~mel/postings/stalls-20130419/log.tar.gz and there
is a lot of stuff in there. The unix time the test started is in the
first line of the file tests-timestamp-bisect . This can help figure out
how far into the test some of the other timestamped logs are
The kernel log with information from the lock_buffer debugging patch is
in dmesg-bisect-gitcheckout . The information in it is race-prone and
cannot be 100% trusted but it's still useful.
iostat is in iostat-bisect-gitcheckout
Here are a few observations I got from the data.
1. flushers appear to acquire buffer locks *very* quickly after an
application writes. Look for lines that look like
"foo failed trylock without holder released 0 ms ago acquired 0 ms ago by bar"
There are a lot of entries like this
jbd2 239 flush-8:0 failed trylock without holder released 0 ms ago acquired 0 ms ago by 2124 tar
jbd2 239 flush-8:0 failed trylock without holder released 0 ms ago acquired 0 ms ago by 2124 tar
jbd2 2124 tar failed trylock without holder released 0 ms ago acquired 0 ms ago by 239 flush-8:0
jbd2 239 flush-8:0 failed trylock without holder released 0 ms ago acquired 0 ms ago by 2124 tar
jbd2 239 flush-8:0 failed trylock without holder released 0 ms ago acquired 0 ms ago by 2124 tar
jbd2 239 flush-8:0 failed trylock without holder released 0 ms ago acquired 0 ms ago by 2124 tar
jbd2 239 flush-8:0 failed trylock without holder released 0 ms ago acquired 0 ms ago by 2124 tar
I expected flushers to be writing back the buffers just released in about
5 seconds time, not immediately. It may indicate that when flushers
wake to clean expired inodes that it keeps cleaning inodes as they are
being dirtied.
2. The flush thread can prevent a process making forward progress for
a long time. Take this as an example
jbd2 stalled dev 8,8 for 8168 ms lock holdtime 20692 ms
Last Owner 239 flush-8:0 Acquired Stack
[<ffffffff8100fd8a>] save_stack_trace+0x2a/0x50
[<ffffffff811a3ad6>] set_lock_buffer_owner+0x86/0x90
[<ffffffff811a72ee>] __block_write_full_page+0x16e/0x360
[<ffffffff811a75b3>] block_write_full_page_endio+0xd3/0x110
[<ffffffff811a7600>] block_write_full_page+0x10/0x20
[<ffffffff811aa7f3>] blkdev_writepage+0x13/0x20
[<ffffffff81119352>] __writepage+0x12/0x40
[<ffffffff81119b56>] write_cache_pages+0x206/0x460
[<ffffffff81119df5>] generic_writepages+0x45/0x70
[<ffffffff8111accb>] do_writepages+0x1b/0x30
[<ffffffff81199d60>] __writeback_single_inode+0x40/0x1b0
[<ffffffff8119c40a>] writeback_sb_inodes+0x19a/0x350
[<ffffffff8119c656>] __writeback_inodes_wb+0x96/0xc0
[<ffffffff8119c8fb>] wb_writeback+0x27b/0x330
[<ffffffff8119e300>] wb_do_writeback+0x190/0x1d0
[<ffffffff8119e3c3>] bdi_writeback_thread+0x83/0x280
[<ffffffff8106901b>] kthread+0xbb/0xc0
[<ffffffff8159e1fc>] ret_from_fork+0x7c/0xb0
[<ffffffffffffffff>] 0xffffffffffffffff
This part is saying that we locked the buffer due to blkdev_writepage
which I assume must be a metadata update. Based on where we lock the
buffer, the only reason we would leave the buffer unlocked if this
was an asynchronous write request leaving the buffer to be unlocked by
end_buffer_async_write at some time in the future
Last Owner Activity Stack: 239 flush-8:0
[<ffffffff812aee61>] __blkdev_issue_zeroout+0x191/0x1a0
[<ffffffff812aef51>] blkdev_issue_zeroout+0xe1/0xf0
[<ffffffff8121abe9>] ext4_ext_zeroout.isra.30+0x49/0x60
[<ffffffff8121ee47>] ext4_ext_convert_to_initialized+0x227/0x5f0
[<ffffffff8121f8a3>] ext4_ext_handle_uninitialized_extents+0x2f3/0x3a0
[<ffffffff8121ff57>] ext4_ext_map_blocks+0x5d7/0xa00
[<ffffffff811f0715>] ext4_map_blocks+0x2d5/0x470
[<ffffffff811f47da>] mpage_da_map_and_submit+0xba/0x2f0
[<ffffffff811f52e0>] ext4_da_writepages+0x380/0x620
[<ffffffff8111accb>] do_writepages+0x1b/0x30
[<ffffffff81199d60>] __writeback_single_inode+0x40/0x1b0
[<ffffffff8119c40a>] writeback_sb_inodes+0x19a/0x350
[<ffffffff8119c656>] __writeback_inodes_wb+0x96/0xc0
[<ffffffff8119c8fb>] wb_writeback+0x27b/0x330
This part is indicating that at the time a process tried to acquire
the buffer lock that flusher was off doing something else entirely.
That points again to the metadata write being asynchronous.
Current Owner 1829 stap
[<ffffffff8100fd8a>] save_stack_trace+0x2a/0x50
[<ffffffff811a3ad6>] set_lock_buffer_owner+0x86/0x90
[<ffffffff8123a5b2>] do_get_write_access+0xd2/0x800
[<ffffffff8123ae2b>] jbd2_journal_get_write_access+0x2b/0x50
[<ffffffff81221249>] __ext4_journal_get_write_access+0x39/0x80
[<ffffffff81229bca>] ext4_free_blocks+0x36a/0xbe0
[<ffffffff8121c686>] ext4_remove_blocks+0x256/0x2d0
[<ffffffff8121c905>] ext4_ext_rm_leaf+0x205/0x520
[<ffffffff8121e64c>] ext4_ext_remove_space+0x4dc/0x750
[<ffffffff8122051b>] ext4_ext_truncate+0x19b/0x1e0
[<ffffffff811efde5>] ext4_truncate.part.61+0xd5/0xf0
[<ffffffff811f0ee4>] ext4_truncate+0x34/0x90
[<ffffffff811f382d>] ext4_setattr+0x18d/0x640
[<ffffffff8118d3e2>] notify_change+0x1f2/0x3c0
[<ffffffff811716f9>] do_truncate+0x59/0xa0
[<ffffffff8117d3f6>] handle_truncate+0x66/0xa0
[<ffffffff81181576>] do_last+0x626/0x820
[<ffffffff81181823>] path_openat+0xb3/0x4a0
[<ffffffff8118237d>] do_filp_open+0x3d/0xa0
[<ffffffff81172869>] do_sys_open+0xf9/0x1e0
[<ffffffff8117296c>] sys_open+0x1c/0x20
[<ffffffff8159e2ad>] system_call_fastpath+0x1a/0x1f
[<ffffffffffffffff>] 0xffffffffffffffff
This is just showing where stap was trying to acquire the buffer lock
truncating data.
3. The blktrace indicates that reads can starve writes from flusher
While there are people that can look at a blktrace and find problems
like they are rain man, I'm more like an ADHD squirrel when looking at
a trace. I wrote a script to look for what unrelated requests completed
while an request got stalled for over a second. It seemed like something
that a tool shoudl already exist for but I didn't find one unless btt
can give the information somehow.
Each delayed request is quite long but here is the first example
discovered by the script
Request 4174 took 1.060828037 to complete
239 W 260608696 [flush-8:0]
Request started time index 4.731862902
Inflight while queued
239 W 260608696 [flush-8:0]
239 W 260608072 [flush-8:0]
239 W 260607872 [flush-8:0]
239 W 260608488 [flush-8:0]
239 W 260608472 [flush-8:0]
239 W 260608568 [flush-8:0]
239 W 260608008 [flush-8:0]
239 W 260607728 [flush-8:0]
239 W 260607112 [flush-8:0]
239 W 260608544 [flush-8:0]
239 W 260622168 [flush-8:0]
239 W 271863816 [flush-8:0]
239 W 260608672 [flush-8:0]
239 W 260607944 [flush-8:0]
239 W 203833687 [flush-8:0]
1676 R 541999743 [watch-inbox-ope]
239 W 260608240 [flush-8:0]
239 W 203851359 [flush-8:0]
239 W 272019768 [flush-8:0]
239 W 260607272 [flush-8:0]
239 W 260607992 [flush-8:0]
239 W 483478791 [flush-8:0]
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239 W 272019776 [flush-8:0]
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239 W 271827160 [flush-8:0]
239 W 260608152 [flush-8:0]
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239 W 260607168 [flush-8:0]
239 W 260608088 [flush-8:0]
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239 W 260608040 [flush-8:0]
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239 W 260608224 [flush-8:0]
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239 W 260607808 [flush-8:0]
239 W 266347440 [flush-8:0]
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239 W 260607512 [flush-8:0]
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239 W 260607424 [flush-8:0]
239 W 260607656 [flush-8:0]
239 W 260607976 [flush-8:0]
239 W 260608440 [flush-8:0]
239 W 260608272 [flush-8:0]
239 W 260607536 [flush-8:0]
239 W 260607920 [flush-8:0]
239 W 260608456 [flush-8:0]
Complete since queueing
1676 R 541999743 [watch-inbox-ope]
239 W 203833687 [flush-8:0]
1676 R 541999759 [watch-inbox-ope]
1676 R 541999791 [watch-inbox-ope]
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239 W 203851359 [flush-8:0]
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1676 R 541952735 [watch-inbox-ope]
1676 R 541952751 [watch-inbox-ope]
1676 R 542112751 [watch-inbox-ope]
1676 R 541952767 [watch-inbox-ope]
1676 R 541952783 [watch-inbox-ope]
1676 R 541952799 [watch-inbox-ope]
1676 R 541952815 [watch-inbox-ope]
1676 R 541952831 [watch-inbox-ope]
1676 R 541952863 [watch-inbox-ope]
1676 R 541952879 [watch-inbox-ope]
1676 R 542113807 [watch-inbox-ope]
1676 R 541952903 [watch-inbox-ope]
1676 R 541952935 [watch-inbox-ope]
1676 RM 540448879 [watch-inbox-ope]
1676 R 541952959 [watch-inbox-ope]
1676 R 542113823 [watch-inbox-ope]
1676 R 542113839 [watch-inbox-ope]
1676 R 542113855 [watch-inbox-ope]
1676 R 541952975 [watch-inbox-ope]
1676 R 541952991 [watch-inbox-ope]
1676 R 541953007 [watch-inbox-ope]
1676 R 541953023 [watch-inbox-ope]
1676 R 541953055 [watch-inbox-ope]
1676 R 541953071 [watch-inbox-ope]
1676 R 541953103 [watch-inbox-ope]
1676 R 541953119 [watch-inbox-ope]
1676 R 541953135 [watch-inbox-ope]
1676 R 542113871 [watch-inbox-ope]
1676 R 542113887 [watch-inbox-ope]
1676 R 541953167 [watch-inbox-ope]
1676 R 541953191 [watch-inbox-ope]
1676 R 541953223 [watch-inbox-ope]
1676 R 541953247 [watch-inbox-ope]
1676 R 541953263 [watch-inbox-ope]
1676 R 541953279 [watch-inbox-ope]
1676 RM 540448887 [watch-inbox-ope]
1676 R 541953303 [watch-inbox-ope]
1676 R 541953327 [watch-inbox-ope]
1676 R 542113903 [watch-inbox-ope]
1676 R 542113919 [watch-inbox-ope]
1676 R 541953359 [watch-inbox-ope]
1676 R 541953375 [watch-inbox-ope]
1676 R 541953391 [watch-inbox-ope]
1676 R 541953407 [watch-inbox-ope]
1676 R 542145679 [watch-inbox-ope]
1676 R 542145695 [watch-inbox-ope]
1676 R 542145711 [watch-inbox-ope]
1676 R 542145727 [watch-inbox-ope]
1676 R 542145743 [watch-inbox-ope]
1676 R 541953423 [watch-inbox-ope]
1676 R 542145759 [watch-inbox-ope]
1676 R 541953455 [watch-inbox-ope]
1676 R 541953471 [watch-inbox-ope]
1676 R 542145775 [watch-inbox-ope]
1676 R 542145791 [watch-inbox-ope]
1676 R 541953487 [watch-inbox-ope]
1676 R 541953519 [watch-inbox-ope]
1676 RM 540448895 [watch-inbox-ope]
1676 R 541953535 [watch-inbox-ope]
1676 R 541953551 [watch-inbox-ope]
1676 R 541953567 [watch-inbox-ope]
1676 R 541953599 [watch-inbox-ope]
1676 R 541953615 [watch-inbox-ope]
1676 R 541953631 [watch-inbox-ope]
1676 R 541953647 [watch-inbox-ope]
1676 R 542157455 [watch-inbox-ope]
1676 R 542157471 [watch-inbox-ope]
1676 R 542157487 [watch-inbox-ope]
1676 R 541953671 [watch-inbox-ope]
1676 RA 540386719 [watch-inbox-ope]
1676 RA 540386727 [watch-inbox-ope]
1676 RA 540386735 [watch-inbox-ope]
1676 RA 540386743 [watch-inbox-ope]
1676 RA 540386751 [watch-inbox-ope]
1676 RA 540386759 [watch-inbox-ope]
1676 RA 540386767 [watch-inbox-ope]
1676 RA 540386775 [watch-inbox-ope]
1676 RA 540386783 [watch-inbox-ope]
1676 RA 540386791 [watch-inbox-ope]
1676 RA 540386799 [watch-inbox-ope]
1676 RA 540386807 [watch-inbox-ope]
1676 RA 540386815 [watch-inbox-ope]
1676 RA 540386823 [watch-inbox-ope]
1676 RA 540386831 [watch-inbox-ope]
1676 RA 540386839 [watch-inbox-ope]
1676 RA 540386847 [watch-inbox-ope]
1676 RA 540386855 [watch-inbox-ope]
1676 RA 540386863 [watch-inbox-ope]
1676 RA 540386871 [watch-inbox-ope]
1676 RA 540386879 [watch-inbox-ope]
1676 RA 540386887 [watch-inbox-ope]
1676 RA 540386895 [watch-inbox-ope]
1676 RA 540386903 [watch-inbox-ope]
1676 RA 540386911 [watch-inbox-ope]
1676 RA 540386919 [watch-inbox-ope]
1676 RA 540386927 [watch-inbox-ope]
1676 RA 540386935 [watch-inbox-ope]
1676 RA 540386943 [watch-inbox-ope]
1676 RA 540386951 [watch-inbox-ope]
1676 RA 540386959 [watch-inbox-ope]
1676 RM 540386711 [watch-inbox-ope]
239 W 271863816 [flush-8:0]
239 W 272019768 [flush-8:0]
239 W 272019776 [flush-8:0]
239 W 483478791 [flush-8:0]
239 W 260578312 [flush-8:0]
239 W 260578400 [flush-8:0]
1676 R 541953695 [watch-inbox-ope]
1676 R 541953711 [watch-inbox-ope]
1676 R 542157503 [watch-inbox-ope]
1676 R 541953743 [watch-inbox-ope]
1676 R 541953759 [watch-inbox-ope]
1676 R 541953775 [watch-inbox-ope]
1676 R 542157519 [watch-inbox-ope]
1676 R 541953791 [watch-inbox-ope]
1676 R 542157551 [watch-inbox-ope]
1676 R 541953807 [watch-inbox-ope]
1676 R 541953831 [watch-inbox-ope]
1676 R 541953863 [watch-inbox-ope]
1676 R 541953927 [watch-inbox-ope]
1676 R 541954055 [watch-inbox-ope]
1676 RM 540448903 [watch-inbox-ope]
1676 R 542157567 [watch-inbox-ope]
1676 R 541954127 [watch-inbox-ope]
1676 R 541954143 [watch-inbox-ope]
1676 R 541954159 [watch-inbox-ope]
1676 R 541954183 [watch-inbox-ope]
1676 R 541954207 [watch-inbox-ope]
1676 R 541954223 [watch-inbox-ope]
1676 R 541954239 [watch-inbox-ope]
1676 R 541954255 [watch-inbox-ope]
1676 R 541954271 [watch-inbox-ope]
1676 R 541954287 [watch-inbox-ope]
1676 R 541954319 [watch-inbox-ope]
1676 R 541954335 [watch-inbox-ope]
1676 R 541954351 [watch-inbox-ope]
1676 R 541954367 [watch-inbox-ope]
1676 R 541954391 [watch-inbox-ope]
1676 R 541954415 [watch-inbox-ope]
1676 R 541954431 [watch-inbox-ope]
1676 R 541954455 [watch-inbox-ope]
1676 R 541954479 [watch-inbox-ope]
1676 R 541954495 [watch-inbox-ope]
1676 RM 540456719 [watch-inbox-ope]
239 W 260622168 [flush-8:0]
239 W 260625528 [flush-8:0]
239 W 260625608 [flush-8:0]
239 W 260614368 [flush-8:0]
239 W 260614336 [flush-8:0]
239 W 260614304 [flush-8:0]
239 W 260614280 [flush-8:0]
1676 R 541954511 [watch-inbox-ope]
1676 R 541954527 [watch-inbox-ope]
1676 R 541954543 [watch-inbox-ope]
1676 R 541954567 [watch-inbox-ope]
1676 R 541954599 [watch-inbox-ope]
1676 R 541954607 [watch-inbox-ope]
1676 R 541954623 [watch-inbox-ope]
1676 R 541954655 [watch-inbox-ope]
1676 R 541954671 [watch-inbox-ope]
1676 R 541954687 [watch-inbox-ope]
1676 R 542158351 [watch-inbox-ope]
1676 R 542158367 [watch-inbox-ope]
1676 R 542158383 [watch-inbox-ope]
1676 R 542158399 [watch-inbox-ope]
1676 R 542158415 [watch-inbox-ope]
1676 R 541954703 [watch-inbox-ope]
1676 R 541954727 [watch-inbox-ope]
1676 R 541954751 [watch-inbox-ope]
1676 R 542158431 [watch-inbox-ope]
1676 R 542158447 [watch-inbox-ope]
1676 R 542158463 [watch-inbox-ope]
1676 R 541954767 [watch-inbox-ope]
1676 RM 540456727 [watch-inbox-ope]
1676 R 541954831 [watch-inbox-ope]
1676 R 541954863 [watch-inbox-ope]
1676 R 541954783 [watch-inbox-ope]
1676 R 541954799 [watch-inbox-ope]
1676 R 541954895 [watch-inbox-ope]
1676 R 541954911 [watch-inbox-ope]
1676 R 541954927 [watch-inbox-ope]
1676 R 541954943 [watch-inbox-ope]
1676 R 542158479 [watch-inbox-ope]
1676 R 541954959 [watch-inbox-ope]
1676 R 542158495 [watch-inbox-ope]
1676 R 541954983 [watch-inbox-ope]
1676 R 541955007 [watch-inbox-ope]
1676 R 541955023 [watch-inbox-ope]
1676 R 541955047 [watch-inbox-ope]
1676 R 541955071 [watch-inbox-ope]
1676 R 541955087 [watch-inbox-ope]
1676 R 541955119 [watch-inbox-ope]
1676 R 541955183 [watch-inbox-ope]
239 W 260607112 [flush-8:0]
239 W 260607152 [flush-8:0]
239 W 260607168 [flush-8:0]
239 W 260607192 [flush-8:0]
239 W 260607208 [flush-8:0]
239 W 260607272 [flush-8:0]
239 W 260607320 [flush-8:0]
239 W 260607384 [flush-8:0]
239 W 260607424 [flush-8:0]
239 W 260607440 [flush-8:0]
239 W 260607456 [flush-8:0]
239 W 260607480 [flush-8:0]
239 W 260607512 [flush-8:0]
239 W 260607536 [flush-8:0]
239 W 260607568 [flush-8:0]
239 W 260607632 [flush-8:0]
239 W 260607656 [flush-8:0]
239 W 260607680 [flush-8:0]
239 W 260607704 [flush-8:0]
239 W 260607728 [flush-8:0]
239 W 260607744 [flush-8:0]
239 W 260607776 [flush-8:0]
239 W 260607808 [flush-8:0]
239 W 260607856 [flush-8:0]
239 W 260607872 [flush-8:0]
239 W 260607888 [flush-8:0]
239 W 260607920 [flush-8:0]
239 W 260607944 [flush-8:0]
239 W 260607976 [flush-8:0]
239 W 260607992 [flush-8:0]
239 W 260608008 [flush-8:0]
239 W 260608040 [flush-8:0]
239 W 260608056 [flush-8:0]
239 W 260608072 [flush-8:0]
239 W 260608088 [flush-8:0]
239 W 260608152 [flush-8:0]
239 W 260608200 [flush-8:0]
239 W 260608224 [flush-8:0]
239 W 260608240 [flush-8:0]
239 W 260608272 [flush-8:0]
239 W 260608312 [flush-8:0]
239 W 260608352 [flush-8:0]
239 W 260608400 [flush-8:0]
239 W 260608424 [flush-8:0]
239 W 260608440 [flush-8:0]
1676 R 541955311 [watch-inbox-ope]
1676 R 541955327 [watch-inbox-ope]
1676 R 542158511 [watch-inbox-ope]
1676 R 542158543 [watch-inbox-ope]
1676 R 542158559 [watch-inbox-ope]
1676 RM 540456735 [watch-inbox-ope]
1676 R 542158575 [watch-inbox-ope]
1676 R 542158591 [watch-inbox-ope]
1676 R 541955343 [watch-inbox-ope]
1676 R 541955359 [watch-inbox-ope]
1676 R 541955375 [watch-inbox-ope]
1676 R 541955391 [watch-inbox-ope]
1676 R 541955423 [watch-inbox-ope]
1676 R 541955439 [watch-inbox-ope]
1676 R 542187279 [watch-inbox-ope]
239 W 260608456 [flush-8:0]
239 W 260608472 [flush-8:0]
239 W 260608488 [flush-8:0]
239 W 260608528 [flush-8:0]
239 W 260608544 [flush-8:0]
239 W 260608568 [flush-8:0]
239 W 260608584 [flush-8:0]
239 W 260608600 [flush-8:0]
239 W 260608632 [flush-8:0]
239 W 260608672 [flush-8:0]
1676 R 542187295 [watch-inbox-ope]
1676 R 542187311 [watch-inbox-ope]
1676 R 541955463 [watch-inbox-ope]
1676 R 541955503 [watch-inbox-ope]
1676 R 541955487 [watch-inbox-ope]
1676 R 542187327 [watch-inbox-ope]
1676 R 541955535 [watch-inbox-ope]
1676 R 541955551 [watch-inbox-ope]
1676 R 541955567 [watch-inbox-ope]
1676 R 541955583 [watch-inbox-ope]
1676 R 541955615 [watch-inbox-ope]
1676 R 541955655 [watch-inbox-ope]
1676 RM 540456743 [watch-inbox-ope]
1676 R 541955679 [watch-inbox-ope]
1676 R 541955695 [watch-inbox-ope]
1676 R 541955711 [watch-inbox-ope]
1676 R 542187343 [watch-inbox-ope]
1676 R 542187359 [watch-inbox-ope]
1676 R 542187375 [watch-inbox-ope]
1676 R 542187391 [watch-inbox-ope]
1676 R 542190479 [watch-inbox-ope]
1676 R 541955727 [watch-inbox-ope]
1676 R 541955751 [watch-inbox-ope]
1676 R 541955775 [watch-inbox-ope]
1676 R 542190495 [watch-inbox-ope]
1676 R 542190511 [watch-inbox-ope]
1676 R 542190527 [watch-inbox-ope]
1676 R 541955791 [watch-inbox-ope]
1676 R 541955823 [watch-inbox-ope]
1676 R 541955839 [watch-inbox-ope]
1676 R 541955855 [watch-inbox-ope]
1676 R 541955879 [watch-inbox-ope]
1676 R 541955903 [watch-inbox-ope]
1676 R 541955919 [watch-inbox-ope]
1676 RM 540456751 [watch-inbox-ope]
1676 R 541955943 [watch-inbox-ope]
1676 R 541955967 [watch-inbox-ope]
1676 R 542190543 [watch-inbox-ope]
1676 R 542190559 [watch-inbox-ope]
1676 R 542190575 [watch-inbox-ope]
1676 R 542190591 [watch-inbox-ope]
1676 R 542193807 [watch-inbox-ope]
1676 R 541955983 [watch-inbox-ope]
1676 R 541956015 [watch-inbox-ope]
1676 R 541956031 [watch-inbox-ope]
1676 R 541956047 [watch-inbox-ope]
1676 R 541956079 [watch-inbox-ope]
1676 R 541956095 [watch-inbox-ope]
1676 R 542193839 [watch-inbox-ope]
1676 R 542193855 [watch-inbox-ope]
1676 R 542193871 [watch-inbox-ope]
1676 R 541956111 [watch-inbox-ope]
1676 R 541956143 [watch-inbox-ope]
1676 R 541956207 [watch-inbox-ope]
1676 R 541956255 [watch-inbox-ope]
1676 R 542193887 [watch-inbox-ope]
1676 R 541956271 [watch-inbox-ope]
1676 R 541956287 [watch-inbox-ope]
1676 R 541956335 [watch-inbox-ope]
1676 RM 540456759 [watch-inbox-ope]
1676 R 542193903 [watch-inbox-ope]
1676 R 541956319 [watch-inbox-ope]
1676 R 541956367 [watch-inbox-ope]
1676 R 541956399 [watch-inbox-ope]
1676 R 541956415 [watch-inbox-ope]
1676 R 541956431 [watch-inbox-ope]
1676 R 541956447 [watch-inbox-ope]
1676 R 541956479 [watch-inbox-ope]
1676 R 542197775 [watch-inbox-ope]
1676 R 542197791 [watch-inbox-ope]
1676 R 542197807 [watch-inbox-ope]
1676 R 542197823 [watch-inbox-ope]
1676 R 542197839 [watch-inbox-ope]
I recognise that the output will have a WTF reaction but the key
observations to me are
a) a single write request from flusher took over a second to complete
b) at the time it was queued, it was mostly other writes that were in
the queue at the same time
c) The write request and the parallel writes were all asynchronous write
requests
D) at the time the request completed, there were a LARGE number of
other requested queued and completed at the same time.
Of the requests queued and completed in the meantime the breakdown was
22 RM
31 RA
82 W
445 R
If I'm reading this correctly, it is saying that 22 reads were merged (RM),
31 reads were remapped to another device (RA) which is probably reads from
the dm-crypt partition, 82 were writes (W) which is not far off the number
of writes that were in the queue and 445 were other reads. The delay was
dominated by reads that were queued after the write request and completed
before it.
There are lots of other example but here is part of one from much later
that starts with.
Request 27128 took 7.536721619 to complete
239 W 188973663 [flush-8:0]
That's saying that the 27128th request in the trace took over 7 seconds
to complete and was an asynchronous write from flusher. The contents of
the queue are displayed at that time and the breakdown of requests is
23 WS
86 RM
124 RA
442 W
1931 R
7 seconds later when it was completed the breakdown of completed
requests was
25 WS
114 RM
155 RA
408 W
2457 R
In combination, that confirms for me that asynchronous writes from flush
are being starved by reads. When a process requires a buffer that is locked
by that asynchronous write from flusher, it stalls.
> The thing is, we do want to make ext4 work well with cfq, and
> prioritizing non-readahead read requests ahead of data writeback does
> make sense. The issue is with is that metadata writes going through
> the block device could in some cases effectively cause a priority
> inversion when what had previously been an asynchronous writeback
> starts blocking a foreground, user-visible process.
>
> At least, that's the theory;
I *think* the data more or less confirms the theory but it'd be nice if
someone else double checked in case I'm seeing what I want to see
instead of what is actually there.
--
Mel Gorman
SUSE Labs
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