[Lustre-discuss] Lustre buffer cache causes large system overhead.

Carlson, Timothy S Timothy.Carlson at pnnl.gov
Thu Aug 22 07:40:01 PDT 2013

FWIW, we have seen the same issues with Lustre 1.8.x and slightly older RHEL6 kernel.  We do the "echo" as part of our slurm prolog/epilog scripts. Not a fix but a workaround before/after jobs run.  No swap activity, but very large buffer cache in use. 


-----Original Message-----
From: lustre-discuss-bounces at lists.lustre.org [mailto:lustre-discuss-bounces at lists.lustre.org] On Behalf Of Roger Sersted
Sent: Thursday, August 22, 2013 7:22 AM
To: lustre-discuss at lists.lustre.org
Cc: Roy Dragseth
Subject: Re: [Lustre-discuss] Lustre buffer cache causes large system overhead.

Is this slowdown due to increased swap activity?  If "yes", then try lowering the "swappiness" value.  This will sacrifice buffer cache space to lower swap activity.

Take a look at http://en.wikipedia.org/wiki/Swappiness.

Roger S.

On 08/22/2013 08:51 AM, Roy Dragseth wrote:
> We have just discovered that a large buffer cache generated from 
> traversing a lustre file system will cause a significant system 
> overhead for applications with high memory demands.  We have seen a 
> 50% slowdown or worse for applications.  Even High Performance 
> Linpack, that have no file IO whatsoever is affected.  The only remedy 
> seems to be to empty the buffer cache from memory by running "echo 3 > /proc/sys/vm/drop_caches"
> Any hints on how to improve the situation is greatly appreciated.
> System setup:
> Client: Dual socket Sandy Bridge, with 32GB ram and infiniband 
> connection to lustre server.  CentOS 6.4, with kernel 
> 2.6.32-358.11.1.el6.x86_64 and lustre
> v2.1.6 rpms downloaded from whamcloud download site.
> Lustre: 1 MDS and 4 OSS running Lustre 2.1.3 (also from whamcloud 
> site).  Each OSS has 12 OST, total 1.1 PB storage.
> How to reproduce:
> Traverse the lustre file system until the buffer cache is large 
> enough.  In our case we run
>   find . -print0 -type f | xargs -0 cat > /dev/null
> on the client until the buffer cache reaches ~15-20GB.  (The lustre 
> file system has lots of small files so this takes up to an hour.)
> Kill the find process and start a single node parallel application, we 
> use HPL (high performance linpack).  We run on all 16 cores on the 
> system with 1GB ram per core (a normal run should complete in appr. 
> 150 seconds.)  The system monitoring shows a 10-20% system cpu 
> overhead and the HPL run takes more than
> 200 secs.  After running "echo 3 > /proc/sys/vm/drop_caches" the 
> system performance goes back to normal with a run time at 150 secs.
> I've created an infographic from our ganglia graphs for the above scenario.
> https://dl.dropboxusercontent.com/u/23468442/misc/lustre_bc_overhead.p
> ng
> Attached is an excerpt from perf top indicating that the kernel 
> routine taking the most time is _spin_lock_irqsave if that means anything to anyone.
> Things tested:
> It does not seem to matter if we mount lustre over infiniband or ethernet.
> Filling the buffer cache with files from an NFS filesystem does not 
> degrade performance.
> Filling the buffer cache with one large file does not give degraded performance.
> (tested with iozone)
> Again, any hints on how to proceed is greatly appreciated.
> Best regards,
> Roy.
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