[Linux-cluster] optimising DLM speed?

Marc Grimme grimme at atix.de
Thu Feb 24 13:24:04 UTC 2011

This is what I also observed.

The change of the different hashtable sizes didn't change the performance.

In my opinion another question is if the hash function is really good or if it does not create bucketlists of same size but few with long list holding all locks. Then a bigger size of the dlm hash tables is irrelevant.

Is there any way to examine how the buckets are being utilized?


----- "Alan Brown" <ajb2 at mssl.ucl.ac.uk> schrieb:

> After running several days with the larger table sizes I don't think 
> it's made any difference to individual thread performance or overall 
> throughput.
> Likewise, the following changes have had no effect on access time for
> large directories (but they have improved caching and improved high
> load 
> overall performance):
> Increasing dentry and inode caches to the maximum size allowed by the
> kernel (about 128million entries. This is limited as a percentage of 
> memory to about 10%)
> This helped caching under load, but until I added the following
> change:
> (sysctl)
> vm.max_reclaims_in_progress=1
> vm.zone_reclaim_mode=0
> The cached dentry data would evaporate after a while.
> (Switching to reeclaim_mode=0 is recommended for fileservers to
> enhance 
> dentry/inode caching)
> At the end of all that, the effect is only minor and the biggest
> bugbear 
> - access to directories with more than ~150 files onboard is unusably
> slow - hasn't been addressed.
> The change which had the largest effect on this problem - switching to
> lock_nolock - isn't practical in a production cluster environment (and
> defeats the purpose of using GFS2 anyway)
> Iostat's showing that under heavy i/o load (1000-3000 requests/second
> but only 2-3Mb/s actual data), the kernel on one machine can sit on 
> read/write equests for up to 3000ms before passing them to the storage
> devices - which usually respond within 2-5ms. It's sitting at 300ms
> most 
> of the time and the machine concerned only has 5 FSes mounted.
> The other 2 machines in the cluster not facing this kind of 
> treatement(100-300 requests/second) have 30 mounts each, can easily
> read 
> at 10-20Mb/s and have read delays of 2-10ms (mostly 3-4).
> Users report that these 2 machines are _fast_ when not accessing 
> directories with large numbers of files onboard...
> --
> Linux-cluster mailing list
> Linux-cluster at redhat.com
> https://www.redhat.com/mailman/listinfo/linux-cluster


Marc Grimme

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