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Re: [Linux-cluster] optimising DLM speed?



Hi Steve,
I think lately I observed a very similar behavior with RHEL5 and gfs2.
It was a gfs2 filesystem that had about 2Mio files with sum of 2GB in a directory. When I did a du -shx . in this directory it took about 5 Minutes (noatime mountoption given). Independently on how much nodes took part in the cluster (in the end I only tested with one node). This was only for the first time running all later executed du commands were much faster.
When I mounted the exact same filesystem with lockproto=lock_nolock it took about 10-20 seconds to proceed with the same command.

Next I started to analyze this with oprofile and observed the following result:

opreport --long-file-names:
CPU: AMD64 family10, speed 2900.11 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a unit mask of 0x00 (No unit mask) count 100000
samples  %        symbol name
200569   46.7639  search_rsb_list
118905   27.7234  create_lkb
32499     7.5773  search_bucket
4125      0.9618  find_lkb
3641      0.8489  process_send_sockets
3420      0.7974  dlm_scan_rsbs
3184      0.7424  _request_lock
3012      0.7023  find_rsb
2735      0.6377  receive_from_sock
2610      0.6085  _receive_message
2543      0.5929  dlm_allocate_rsb
2299      0.5360  dlm_hash2nodeid
2228      0.5195  _create_message
2180      0.5083  dlm_astd
2163      0.5043  dlm_find_lockspace_global
2109      0.4917  dlm_find_lockspace_local
2074      0.4836  dlm_lowcomms_get_buffer
2060      0.4803  dlm_lock
1982      0.4621  put_rsb
..

opreport --image /gfs2
CPU: AMD64 family10, speed 2900.11 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a unit mask of 0x00 (No unit mask) count 100000
samples  %        symbol name
9310     15.5600  search_bucket
6268     10.4758  do_promote
2704      4.5192  gfs2_glock_put
2289      3.8256  gfs2_glock_hold
2286      3.8206  gfs2_glock_schedule_for_reclaim
2204      3.6836  gfs2_glock_nq
2204      3.6836  run_queue
2001      3.3443  gfs2_holder_wake
..

opreport --image /dlm
CPU: AMD64 family10, speed 2900.11 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a unit mask of 0x00 (No unit mask) count 100000
samples  %        symbol name
200569   46.7639  search_rsb_list
118905   27.7234  create_lkb
32499     7.5773  search_bucket
4125      0.9618  find_lkb
3641      0.8489  process_send_sockets
3420      0.7974  dlm_scan_rsbs
3184      0.7424  _request_lock
3012      0.7023  find_rsb
2735      0.6377  receive_from_sock
2610      0.6085  _receive_message
2543      0.5929  dlm_allocate_rsb
2299      0.5360  dlm_hash2nodeid
2228      0.5195  _create_message
..

This very much reminded me on a similar test we've done years ago with gfs (see http://www.open-sharedroot.org/Members/marc/blog/blog-on-dlm/red-hat-dlm-__find_lock_by_id/profile-data-with-diffrent-table-sizes).

Does this not show that during the du command 46% of the time the kernel stays in the dlm:search_rsb_list function while looking out for locks. It still looks like the hashtable for the lock in dlm is much too small and searching inside the hashmap is not constant anymore?

I would be really interesting how long the described backup takes when the gfs2 filesystem is mounted exclusively on one node without locking.
For me it looks like you're facing a similar problem with gfs2 that has been worked around with gfs by introducing the glock_purge functionality that leads to a much smaller glock->dlm->hashtable and makes backups and the like much faster.

I hope this helps.

Thanks and regards
Marc.

----- Original Message -----
From: "Steven Whitehouse" <swhiteho redhat com>
To: "linux clustering" <linux-cluster redhat com>
Sent: Dienstag, 15. Februar 2011 19:20:20
Subject: Re: [Linux-cluster] optimising DLM speed?

Hi,

On Tue, 2011-02-15 at 17:59 +0000, Alan Brown wrote:
> After lots of headbanging, I'm slowly realising that limits on GFS2 lock 
> rates and totem message passing appears to be the main inhibitor of 
> cluster performance.
> 
> Even on disks which are only mounted on one node (using lock_dlm), the 
> ping_pong rate is - quite frankly - appalling, at about 5000 
> locks/second, falling off to single digits when 3 nodes are active on 
> the same directory.
> 
Let me try and explain what is going on here.... the posix (fcntl) locks
which you are using, do not go through the dlm, or at least not the main
part of the dlm.

The lock requests are sent to either gfs_controld or dlm_controld,
depending upon the version of RHCS where the requests are processed in
userspace via corosync/openais.

> totem's defaults are pretty low:
> 
> (from man openais.conf)
> 
> max messages/second = 17
> window_size = 50
> encryption = on
> encryption/decryption threads = 1
> netmtu = 1500
> 
> I suspect tuning these would have a marked effect on performance
> 
> gfs_controld and dlm_controld aren't even appearing in the CPU usage 
> tables (24Gb dual 5560CPUs)
> 
Only one of gfs_controld/dlm_controld will have any part in dealing with
the locks that you are concerned with, depending on the version.

> We have 2 GFS clusters, 2 nodes (imap) and 3 nodes (fileserving)
> 
> The imap system has around 2.5-3 million small files in the Maildir imap 
> tree, whilst the fileserver cluster has ~90 1Tb filesystems of 1-4 
> million files apiece (fileserver total is around 150 million files)
> 
> When things get busy or when users get silly and drop 10,000 files in a 
> directory, performance across the entire cluster goes downhill badly - 
> not just in the affected disk or directory.
> 
> Even worse: backups - it takes 20-28 hours to run a 0 file incremental 
> backup of a 2.1million file system (ext4 takes about 8 minutes for the 
> same file set!)
> 
The issues you've reported here don't sound to me as if they are related
to the rate of posix locks which can be granted. These sound to me a lot
more like issues relating to the I/O pattern on the filesystem.

How is the data spread out across directories and across nodes? Do you
try to keep users local to a single node for the imap servers? Is the
backup just doing a single pass scan over the whole fileystem?

> 
> All heartbeat/lock traffic is handled across a dedicated Gb switch with 
> each cluster in its own vlan to ensure no external cruft gets in to 
> cause problems.
> 
> I'm seeing heartbeat/lock lan traffic peak out at about 120kb/s and 
> 4000pps per node at the moment. Clearly the switch isn't the problem - 
> and using hardware acclerated igb devices I'm pretty sure the 
> networking's fine too.
> 
During the actual workload, or just during the ping pong test?

Steve.

> SAN side, there are 4 8Gb Qlogic cards facing the fabric and right now 
> the whole mess talks to a Nexsan atabeast (which is slow, but seldom 
> gets its commmand queue maxed out.)
> 
> Has anyone played much with the totem message timings? if so what 
> results have you had?
> 
> As a comparison, the same hardware using EXT4 on a standalone system can 
> trivially max out multiple 1Gb/s interfaces while transferring 1-2Mb/s 
> files and gives lock rates of 1.8-2.5 million locks/second even with 
> multiple ping_pong processes running.
> 
> 
> 
> --
> Linux-cluster mailing list
> Linux-cluster redhat com
> https://www.redhat.com/mailman/listinfo/linux-cluster


--
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-- 

Marc Grimme

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Fax: +49 89 9901766-0
E-Mail: grimme atix de

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