On Fri, Jul 31, 2020 at 01:27:31PM +0200, Donald Buczek wrote: > Dear Linux people, > > we have a backup server with two xfs filesystems on 101.9TB md-raid6 devices (16 * 7.3 T disks) each, Current Linux version is 5.4.54. ..... > root:done:/home/buczek/linux_problems/shrinker_semaphore/# cat /proc/meminfo > MemTotal: 263572332 kB 256GB of RAM. > MemFree: 2872368 kB > MemAvailable: 204193824 kB 200GB "available" > Buffers: 2568 kB > Cached: 164931356 kB 160GB in page cache > KReclaimable: 40079660 kB > Slab: 49988268 kB > SReclaimable: 40079660 kB 40GB in reclaimable slab objects. IOWs, you have no free memory in the machine and so allocation will frequently be dipping into memory reclaim to free up page cache and slab caches to make memory available. > xfs_inode 30978282 31196832 960 4 1 : tunables 54 27 8 : slabdata 7799208 7799208 434 Yes, 30 million cached inodes. > bio_integrity_payload 29644966 30203481 192 21 1 : tunables 120 60 8 : slabdata 1438261 1438261 480 Either there is a memory leak in this slab, or it is shared with something like the xfs_ili slab, which would indicate that most of the cached inodes have been dirtied in memory at some point in time. 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 permaturely. 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/md0 isize=512 agcount=102, agsize=268435328 blks > = sectsz=4096 attr=2, projid32bit=1 > = crc=1 finobt=1, sparse=1, rmapbt=0 > = reflink=0 > data = bsize=4096 blocks=27348629504, imaxpct=1 > = sunit=128 swidth=1792 blks > naming =version 2 bsize=4096 ascii-ci=0, ftype=1 > log =internal log bsize=4096 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... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
