Excessive slab use deteriorates performance

We have 2 almost identical servers, with the following difference:
1 16GB RAM, 4 disks in RAID10
2 8GB RAM, 2 disks in RAID1, but tried also in RAID0

The 2nd machine actually started life as a restore of a snapshot of the first, so running much the same services except the one we intentionally disabled.

We have noticed on both machines that after a day of up-time logging into KDE (as well as other file intensive tasks, like apt) are extremely slow (especially compared to the desktop/laptop machines with slower CPU, single disk).

It seems that actually the machine with the most memory suffers the most. Looking at the memory use of user space, that is very low (< 1GB), but the total memory consumed is actually almost 100%. That would be fine if it went to file buffers, but it doesn't. It goes to slab, specifically to btrfs_inode, dentry, radix_tree_node, btrfs_extent_buffer, leaving almost no memory available for buffers, or user space.

As far I we know these slabs should be freeed when needed, and there should be no reason to tinker with it. However, we found that:
sync ; echo 2 > /proc/sys/vm/drop_caches # free dentries and inodes restores performance for another day.

Looking into the jobs that run at night, it appears to be caused by the cron script that runs updatedb (to create the database for locate).

In the past (when using ext4) we never saw this problem, but then (in the old days) the server didn't have so much memory.

Could there be any relation to btrfs causing the mentioned slabs to not be automatically freeed?

In the mean time we put:
vfs_cache_pressure = 10000

And this seems to be keeping slabs total at 2.5GB (with btrfs_inode at 1.7GB).

Still, manually doing drop_caches, will reduce slabs total to 0.4GB, with btrfs_inode at 0.01GB.

I'm not sure, but just having a RO operation scanning all files on the disk causing kernel memory to be used to cache things that are almost not used 12 hours later, while keeping more useful file caches from growing, seems not really optimal.

Has anybody else seen this behavior?

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Ferry Toth

Yes, a friend showed me almost identical situtation some time ago. Slab caches full, not reclaimed and dropping caches "fixed" that, likely caused by overnight cron jobs. At that time I was interested whether it was a leak but if the slab usage dropped and did not debug it further.

The sysctl vfs_cache_pressure should help to tune the system. The btrfs allocates slabs with the "reclaimable" flag on so there should not be any direct obstacle for that so there must be something else going on.

In our nightly cron jobs I found locate and mlocate. Now we are suspecting locate and removed that - as we understand mlocate should be a more efficient replacement for mlocate, so that might have been redundant.

Exactly, as it reclaims without trouble when manually dropping the slabs AND
reclaims better when putting vfs pressure excessively high, there might be something wrong either in btrfs or the reclaiming mechanism elsewhere in the
kernel.

But if it is the latter, you would expect ext users to have similar troubles, at least with excessive slabs for dentry.

I would like to report this as a bug, but am not sure where to put it.

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Ferry