Re: Reducing inode cache usage on 2.4?

[James Pearson]

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
>
>
> vm_vfs_scan_ratio:
> ------------------
> 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?

Thanks

James Pearson

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