Re: Reducing inode cache usage on 2.4?

From: James Pearson
Date: Fri Dec 17 2004 - 19:34:47 EST

Marcelo Tosatti wrote:

Or am I looking in completely the wrong place i.e. the inode cache is not the problem?

No, in your case the extreme inode/dcache sizes indeed seem to be a problem.

The default kernel shrinking ratio can be tuned for enhanced reclaim efficiency.

xfs_inode 931428 931428 408 103492 103492 1 : 124 62
dentry_cache 499222 518850 128 17295 17295 1 : 252 126

is what proportion of the VFS queues we will scan in one go.
A value of 6 for vm_vfs_scan_ratio implies that 1/6th of the
unused-inode, dentry and dquot caches will be freed during a
normal aging round.
Big fileservers (NFS, SMB etc.) probably want to set this
value to 3 or 2.

The default value is 6.

Tune /proc/sys/vm/vm_vfs_scan_ratio increasing the value to 10 and so on and examine the results.

Thanks for the info - but doesn't increasing the value of vm_vfs_scan_ratio mean that less of the caches will be freed?

Doing a few tests (on another test file system with 2 million or so files and 1Gb of memory) running 'find $disk -type f', with vm_vfs_scan_ratio set to 6 (or 10), the first two column values for xfs_inode, linvfs_icache and dentry_cache in /proc/slabinfo reach about 900000 and stay around that value, but setting vm_vfs_scan_ratio to 1, then each value still reaches 900000, but then falls to a few thousand and increases up to 900000 and then drop away again and repeats.

This still happens when I cat many large files (100Mb) to /dev/null at the same time as running the find i.e. the inode caches can still reach 90% of the memory before being reclaimed (with vm_vfs_scan_ratio set to 1).

If I stop the find process when the inode caches reach about 90% of the memory, and then start cat'ing the large files, it appears the inode caches are never reclaimed (or longer than it takes to cat 100Gb of data to /dev/null) - is this expected behaviour?

It seems the inode cache has priority over cached file data.

What triggers the 'normal ageing round'? Is it possible to trigger this earlier (at a lower memory usage), or give a higher priority to cached data?


James Pearson

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