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

Patrick Shopbell pls at astro.caltech.edu
Fri Aug 23 13:59:44 PDT 2013

Hi all -
I have watched this thread with much interest, and now I am even
more interested/confused.  :-)

Several months back, we had a very substantial slowdown on our
MDS box. Interactive use of the box was very sluggish, even
though the load was quite low. This was eventually solved by
setting the opposite value for the variable in question:

vm.zone_reclaim_mode = 0

And it was equally dramatic in its solution of our problem - the MDS
started responding normally immediately afterwards. We went ahead
and set the value to zero on all of our NUMA machines. (We are
running Lustre 2.3.)

Clearly, I need to do some reading on Lustre and its various caching
issues. This has been a quite interesting discussion.

Thanks everyone for such a great list.

| Patrick Shopbell               Department of Astronomy             |
| pls at astro.caltech.edu          Mail Code 249-17                    |
| (626) 395-4097                 California Institute of Technology  |
| (626) 568-9352  (FAX)          Pasadena, CA 91125                 |
| WWW: http://www.astro.caltech.edu/~pls/                            |

On 8/23/13 1:08 PM, Dragseth Roy Einar wrote:
> Thanks for the suggestion!  It didn't help, but as I read the documentation on
> vfs_cache_pressure in the kernel docs I noticed the next parameter,
> zone_reclaim_mode, which looked like it might be worth fiddling with.  And what
> do you know, changing it from 0 to 1 made the system overhead vanish
> immediately!
> I must admit I do not completely understand why this helps, but it seems to do
> the trick in my case.  We'll put
> vm.zone_reclaim_mode = 1
> into /etc/sysctl.conf from now on.
> Thanks to all for the hints and comments on this.
> A nice weekend to everyone, mine for sure is going to be...
> r.
> On Friday 23. August 2013 09.36.34 Scott Nolin wrote:
>> You might also try increasing the vfs_cache_pressure.
>> This will reclaim inode and dentry caches faster. Maybe that's the
>> problem, not page caches.
>> To be clear - I have no deep insight into Lustre's use of the client
>> cache, but you said you has lots of small files, which if lustre uses
>> the cache system like other filesystems means it may be inodes/dentries.
>> Filling up the page cache with files like you did in your other tests
>> wouldn't have the same effect. Just my guess here.
>> We had some experience years ago with the opposite sort of problem. We
>> have a big ftp server, and we want to *keep* inode/dentry data in the
>> linux cache, as there are often stupid numbers of files in directories.
>> Files were always flowing through the server, so the page cache would
>> force out the inode cache. Was surprised to find with linux there's no
>> ability to set a fixed inode cache size - the best you can do is
>> "suggest" with the cache pressure tunable.
>> Scott
>> On 8/23/2013 6:29 AM, Dragseth Roy Einar wrote:
>>> I tried to change swapiness from 0 to 95 but it did not have any impact on
>>> the system overhead.
>>> r.
>>> On Thursday 22. August 2013 15.38.37 Dragseth Roy Einar wrote:
>>>> No, I cannot detect any swap activity on the system.
>>>> r.
>>>> On Thursday 22. August 2013 09.21.33 you wrote:
>>>>> 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.pn
>>>>>> g
>>>>>> 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.
>>>>>> _______________________________________________
>>>>>> Lustre-discuss mailing list
>>>>>> Lustre-discuss at lists.lustre.org
>>>>>> http://lists.lustre.org/mailman/listinfo/lustre-discuss

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