[Linux-cluster] Slowness above 500 RRDs
David Teigland
teigland at redhat.com
Tue Jun 12 14:46:52 UTC 2007
On Tue, Jun 12, 2007 at 04:01:04PM +0200, Ferenc Wagner wrote:
> Hi David,
>
> Sorry if all what follows is misguided nonsense. I'm eager to learn...
>
> David Teigland <teigland at redhat.com> writes:
>
> > The new code has much better caching in the dlm which will benefit flocks,
> > look at these flock numbers I sent before: [...]
> >
> > This is testing raw flock performance. The dlm locks for normal file
> > operations should be cached and locally mastered also, so I'm not sure
> > what's causing the long times. Make sure that drop_count is zero again,
> > now it's in sysfs:
> > echo 0 > /sys/fs/gfs/<foo>:<bar>/lock_module/drop_count
> >
> > Also, mount debugfs so we can check some stuff later:
> > mount -t debugfs none /sys/kernel/debug
> >
> > Then run some tests:
> > - mount on nodeA
> > - run the test on nodeA
> > - count locks on nodeA
> > (cat /sys/kernel/debug/dlm/<bar> | grep Master | wc -l)
> > - mount on nodeB (don't do anything on this node)
> > - run the test again on nodeA
> > - count locks on nodeA and nodeB (see above)
> > - mount on nodeC (don't do anything on nodes B or C)
> > - run the test again on nodeA
> > - count locks on nodes A, B and C (see above)
> >
> > We're basically trying to produce the best-case performance from one node,
> > nodeA. That means making sure that nodeA is mastering all locks and doing
> > maximum caching. That's why it's important that we not do anything at all
> > that accesses the fs on nodes B or C, or do any extra mounts/unmounts.
>
> I made all the above tests and composed the reply a long time ago, but
> now, getting back to it after that long time, I decided to satisfy your
> curiosity, behold...
>
> > Plocks will be much slower and are probably not interesting to test, but
> > I'm curious if you added the "-l0" option to gfs_controld? That option
> > turns off the code that intentionally limits the rate of plocks. See the
> > old results again: [...]
>
> Now, that switch makes ALL the difference. With a single node
> switched on, I get results like this (with abbreviated strace -c
> output appended):
>
> without -l0:
>
> filecount=500
> iteration=0 elapsed time=10.444446 s
> iteration=1 elapsed time=9.693618 s
> iteration=2 elapsed time=10.520073 s
> iteration=3 elapsed time=10.521504 s
> iteration=4 elapsed time=10.520183 s
> total elapsed time=51.699824 s
> Process 5265 detached
> % time seconds usecs/call calls errors syscall
> ------ ----------- ----------- --------- --------- ----------------
> 83.27 0.048525 6 7551 read
> 6.73 0.003923 2 2502 fcntl64
> 4.47 0.002606 1 2528 close
> 3.09 0.001801 1 2551 23 open
> 0.74 0.000432 0 2507 write
> 0.71 0.000415 0 5033 mmap2
> 0.41 0.000237 0 12528 3 _llseek
> 0.31 0.000178 0 5001 munmap
> 0.18 0.000107 0 5015 fstat64
> 0.08 0.000049 0 2506 gettimeofday
> 0.00 0.000000 0 16 14 ioctl
> 0.00 0.000000 0 202 182 stat64
> ------ ----------- ----------- --------- --------- ----------------
> 100.00 0.058273 47974 229 total
>
> with -l0:
>
> filecount=500
> iteration=0 elapsed time=5.966146 s
> iteration=1 elapsed time=0.582058 s
> iteration=2 elapsed time=0.528272 s
> iteration=3 elapsed time=0.936438 s
> iteration=4 elapsed time=0.528147 s
> total elapsed time=8.541061 s
> Process 10030 detached
> % time seconds usecs/call calls errors syscall
> ------ ----------- ----------- --------- --------- ----------------
> 57.17 0.016527 2 7551 read
> 21.49 0.006213 2 2528 close
> 8.16 0.002358 1 2502 fcntl64
> 6.59 0.001904 1 2551 23 open
> 2.21 0.000638 0 2507 write
> 1.46 0.000421 0 5033 mmap2
> 0.86 0.000249 249 1 execve
> 0.73 0.000212 0 5001 munmap
> 0.65 0.000187 0 12528 3 _llseek
> 0.57 0.000165 0 5015 fstat64
> 0.12 0.000034 0 2506 gettimeofday
> 0.00 0.000000 0 16 14 ioctl
> 0.00 0.000000 0 202 182 stat64
> ------ ----------- ----------- --------- --------- ----------------
> 100.00 0.028908 47974 229 total
>
> Looks like the bottleneck isn't the explicit locking (be it plock or
> flock), but something else, like the built-in GFS locking.
I'm guessing that these were run with a single node in the cluster? The
second set of numbers (with -l0) wouldn't make much sense otherwise. I
think if you add nodes to the cluster, the -l0 numbers will go up quite a
bit. In the end I expect that flocks are still going to be the fastest
for you.
> Similar dramatic speedup can be achieved (with a single node switched
> on, again), by the lockproto=lock_nolock mount option, even if used
> together with ignore_local_fs. It I understand it right, this
> combination leaves the cluster-wide [pf]locks alone, just eliminates
> the GFS internal locking, which guards the internal consistency of the
> file system (please correct me if I'm wrong).
With nolock there is no cluster (lock_nolock just returns 0 for
everything), so the cluster-wide [pf]locks have zero cost. So this test
doesn't tell you anything.
> What's strange, is that gfs_controld -l0 seems like a perfectly safe
> invocation (what's the catch, ie. why was the artifical limit
> introduced?),
The rate limit was introduced to prevent bad programs from flooding the
network with plock operations. It may not be a very real problem, though,
so we might eventually disable it (-l0) by default.
> still it achieves almost the same speedup like using
> lock_nolock, which would be a disaster with more than one node
> mounting the fs. (Also this trick scales pretty well to 4000 files.)
No, -l0 is not going to give you the performance of nolock. I think you
must have been running with a single node in the cluster. In that case
there are no other nodes to send/recv messages to/from, so the plock
messages are very fast.
> Again, the above tests were done with a single node switched on, and
> I'm not sure whether the results carry over to the real cluster setup,
> will test is soon.
Ah, yep. When you add nodes the plocks will become much slower. Again, I
think you'll have better luck with flocks.
> I didn't touch drop_count either, everything was
> left as default, except for the mount options and the -l option.
>
> Also, I can send the results of the scenario suggested by you, if it's
> still relevant. In short: the locks are always mastered on node A
> only, but the performance is poor nevertheless.
Poor even in the first step when you're just mounting on nodeA?
Dave
More information about the Linux-cluster
mailing list