On 01/25/2018 03:08 PM, Brian Foster wrote:
On Thu, Jan 25, 2018 at 10:50:40AM +0200, Avi Kivity wrote:
On 01/23/2018 07:39 PM, Brian Foster wrote:
On Tue, Jan 23, 2018 at 07:00:31PM +0200, Avi Kivity wrote:
On 01/23/2018 06:47 PM, Brian Foster wrote:
On Tue, Jan 23, 2018 at 06:22:07PM +0200, Avi Kivity wrote:
On 01/23/2018 06:11 PM, Brian Foster wrote:
On Tue, Jan 23, 2018 at 05:45:39PM +0200, Avi Kivity wrote:
On 01/23/2018 05:28 PM, Brian Foster wrote:
On Tue, Jan 23, 2018 at 04:57:03PM +0200, Avi Kivity wrote:
I'm seeing the equivalent[*] of xfs_extent_busy_flush() sleeping in my
beautiful io_submit() calls.
Questions:
- Is it correct that RWF_NOWAIT will not detect the condition that led to
the log being forced?
- If so, can it be fixed?
- Can I do something to reduce the odds of this occurring? larger logs,
more logs, flush more often, resurrect extinct species and sacrifice them to
the xfs gods?
- Can an xfs developer do something? For example, make it RWF_NOWAIT
friendly (if the answer to the first question was "correct")
So RWF_NOWAIT eventually works its way to IOMAP_NOWAIT, which looks like
it skips any write call that would require allocation in
xfs_file_iomap_begin(). The busy flush should only happen in the block
allocation path, so something is missing here. Do you have a backtrace
for the log force you're seeing?
Here's a trace. It's from a kernel that lacks RWF_NOWAIT.
Oh, so the case below is roughly how I would have expected to hit the
flush/wait without RWF_NOWAIT. The latter flag should prevent this, to
answer your first question.
Thanks, that's very encouraging. We are exploring recommending upstream-ish
kernels to users and customers, given their relative stability these days
and aio-related improvements (not to mention the shame of having to admit to
running an old kernel when reporting a problem to an upstream list).
For the follow up question, I think this should only occur when the fs
is fairly low on free space. Is that the case here?
No:
/dev/md0 3.0T 1.2T 1.8T 40% /var/lib/scylla
I'm not sure there's
a specific metric, fwiw, but it's just a matter of attempting an (user
data) allocation that only finds busy extents in the free space btrees
and thus has to the force the log to satisfy the allocation.
What does "busy" mean here? recently freed so we want to force the log to
make sure the extent isn't doubly-allocated? (wild guess)
Recently freed and the transaction that freed the blocks has not yet
been persisted to the on-disk log. A subsequent attempt to allocate
those blocks for user data waits for the transaction to commit to disk
to ensure that the block is not written before the filesystem has
persisted the fact that it has been freed. Otherwise, my understanding
is that if the blocks are written to and the filesystem crashes before
the previous free was persisted, we'd have allowed an overwrite of a
still-used metadata block.
Understood, thanks.
I suppose
running with more free space available would avoid this. I think running
with less in-core log space could indirectly reduce extent busy time,
but that may also have other performance ramifications and so is
probably not a great idea.
At 60%, I hope low free space is not a problem.
Yeah, that seems strange. I wouldn't expect busy extents to be a problem
with that much free space.
The workload creates new files, appends to them, lets them stew for a while,
then deletes them. Maybe something is preventing xfs from seeing non-busy
extents?
Yeah, could be.. perhaps the issue is that despite the large amount of
total free space, the free space is too fragmented to satisfy a
particular allocation request..?
from to extents blocks pct
1 1 2702 2702 0.00
2 3 690 1547 0.00
4 7 115 568 0.00
8 15 60 634 0.00
16 31 63 1457 0.00
32 63 102 4751 0.00
64 127 7940 895365 0.19
128 255 49680 12422100 2.67
256 511 1025 417078 0.09
512 1023 4170 3660771 0.79
1024 2047 2168 3503054 0.75
2048 4095 2567 7729442 1.66
4096 8191 8688 59394413 12.76
8192 16383 310 3100186 0.67
16384 32767 112 2339935 0.50
32768 65535 35 1381122 0.30
65536 131071 8 651391 0.14
131072 262143 2 344196 0.07
524288 1048575 4 2909925 0.62
1048576 2097151 3 3550680 0.76
4194304 8388607 10 82497658 17.72
8388608 16777215 10 158022653 33.94
16777216 24567552 5 122778062 26.37
total free extents 80469
total free blocks 465609690
average free extent size 5786.2
Looks like plenty of free large extents, with most of the free space
completely, unfragmented.
Indeed..
Lots of 16MB-32MB extents, too. 32MB is our allocation hint size, could have
something to do with it.
Most likely. Based on this, it's hard to say for certain why you'd be
running into allocation latency caused by busy extents. Does this
filesystem use the '-o discard' mount option by any chance?
No. We'd love to but have had bad experience + strong recommendation
from this list not to use it.
I suppose it's possible that this was some kind of transient state, or
perhaps only a small set of AGs are affected, etc. It's also possible
this may have been improved in more recent kernels by Christoph's rework
of some of that code. In any event, this would probably require a bit
more runtime analysis to figure out where/why allocations are getting
stalled as such. I'd probably start by looking at the xfs_extent_busy_*
tracepoints (also note that if there's potentially something to be
improved on here, it's more useful to do so against current upstream).
Or you could just move to something that supports RWF_NOWAIT.. ;)
NOWAIT only helps with blocking, not high CPU usage. Even if we moved it
to another thread, it would still take the same time (but granted, our
important thread could still do work, sharing the core with the one
doing the allocation).
Regardless, we are recommending to our users and customers to move to
NOWAIT kernels, but of course some are justifiable cautious. Upstream
kernels are now quite stable, so enterprise distribution kernels don't
provide the same value they used to.
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