[Lustre-discuss] Can't increase effective client read cache

[Nathan Dauchy]

On 09/24/2013 02:14 PM, Carlson, Timothy S wrote:
> I've got an odd situation that I can't seem to fix. 
> 
> My setup is Lustre 1.8.8-wc1 clients on RHEL 6 talking to 1.8.6 servers on RHEL 5.
> 
> My compute nodes have 64 GB of memory and I have a use case where an application has very low memory usage and needs to access a few thousand files in Lustre that range from 10 to 50 MB.  The files are subject to some reuse and it would be advantageous to cache as much of the data as possible.  The default cache for this configuration would be 48GB on the client as that is 75% of memory.   However the client never caches more than about 40GB of data according to /proc/meminfo 
> 
> Even if I tune the cached memory to 64GB the amount of cache in use never goes past 40GB. My current setting is as follows
> 
> # lctl get_param llite.*.max_cached_mb
> llite.olympus-ffff8804069da800.max_cached_mb=64000
> 
> I've also played with some of the VM tunable settings.  Like running vfs_cache_pressure down to 10
> 
> # vm.vfs_cache_pressure = 10
> 
>  In no case do I see more than about 35GB of cache being used.   To do some more testing on this I created a bunch (40) 2G files in Lustre and then copied them to /dev/null on the client. While doing this I ran the fincore tool from http://code.google.com/p/linux-ftools/ to see if the file was still in cache. Once about 40GB of cache was used, the kernel started to drop files from the cache even though there was no memory pressure on the system. 
> 
> If I do the same test with files local to the system, I can fill all the cache to about 61GB before files start getting dropped. 
> 
> Is there some other Lustre tunable on the client that I can twiddle with to make more use of the local memory cache?

Tim,

Another kernel sysctl that might be in play here.  Have you looked at these?

  vm.dirty_background_ratio
  vm.dirty_ratio
  vm.dirty_background_bytes
  vm.dirty_bytes

Those will control at what number of bytes or percentage of memory the
kernel flushes buffer cache.

Hope this helps,
Nathan


> Thanks
> 
> Tim Carlson
> Director, PNNL Institutional Computing
> timothy.carlson at pnnl.gov
> 
>