Re: xfs_reclaim_inodes_ag taking several seconds

From: Donald Buczek
Date: Sat Aug 01 2020 - 14:10:33 EST

On 01.08.20 12:25, Donald Buczek wrote:

So if I understand you correctly, this is expected behavior with this kind of load and conceptual changes are already scheduled for kernel 5.9. I don't understand most of it, but isn't it true that with that planned changes the impact might be better limited to the filesystem, so that the performance of other areas of the system might improve? I'd love to test that with our load, but I don't want to risk our backup data and it would be difficult to produce the same load on a toy system. The patch set is not yet ready to be tested on production data, is it?

So I guess I'll try to put the backup processes into one or more cgroups to limit the memory available for their fs caches and leave some room for unrelated (maintenance) processes. I hope, that makes sense.

Which it doesn't, because it totally ignores what was said before. The affected processes were not waiting for memory but for the shrinker to finish.


Thank you both four your analysis!


And if you have 30 million inodes in memory, and lots of them are
dirty, and the shrinkers are running, then they will be doing
dirty inode writeback to throttle memory reclaim to
ensure it makes progress and doesn't declare OOM and kill processes

You have spinning disks, RAID6. I'm betting that it can only clean a
couple of hundred inodes a second because RAID6 is severely IOP
limited for small writes (like inode clusters). And when you many,
many thousands (maybe millions) of dirty inodes, anything that has
to wait on inode writeback is going to be waiting for some time...

root:done:/home/buczek/linux_problems/shrinker_semaphore/# xfs_info /amd/done/C/C8024
meta-data=/dev/md0ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ isize=512ÂÂÂ agcount=102, agsize=268435328 blks
ÂÂÂÂÂÂÂÂÂ =ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ sectsz=4096Â attr=2, projid32bit=1
dataÂÂÂÂ =ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ bsize=4096ÂÂ blocks=27348629504, imaxpct=1
namingÂÂ =version 2ÂÂÂÂÂÂÂÂÂÂÂÂÂ bsize=4096ÂÂ ascii-ci=0, ftype=1
logÂÂÂÂÂ =internal logÂÂÂÂÂÂÂÂÂÂ bsize=4096ÂÂ blocks=521728, version=2

And full size journals, so the filesystem can hold an awful lot of
active dirty inodes in memory before it starts throttling on a full
journal (think millions of dirty inodes per filesystem)...

So, yeah, this is the classic "in memory operation is orders of
magnitude faster than disk operation" and it all comes crashing down
when something needs to wait for inodes to be written back. The
patchset Darrick pointed you at should fix the shrinker issue, but
it's likely that this will just push the problem to the next
throttling point, which is the journal filling up.

IOWs, I suspect fixing your shrinker problem is only going to make
the overload of dirty inodes in the system behave worse, because
running out of journal space cause *all modifications* to the
filesystem to start taking significant delays while they wait for
inode writeback to free journal space, not just have things
trying to register/unregister shrinkers take delays...



Donald Buczek
Tel: +49 30 8413 1433