On Wed, 11 Jun 2008 15:52:22 -0400
"J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> I'm probably missing something fundamental--why doesn't
> /proc/slab_allocators show any results for size-x where x >= 4096?
>
> Someone's seeing a performance problem with the linux nfs server. One
> of the symptoms is the "size-4096" slab cache seems to be out of
> control. I assumed that meant that memory allocated by kmalloc() might
> be leaking, so figured it might be interesting to turn on
> CONFIG_DEBUG_SLAB_LEAK. As far as I can tell what that does is list
> kmalloc() callers in /proc/slab_allocators. But that doesn't seem to be
> showing any results for size-4096. Can anyone provide a clue?
> Thanks!
>
> --b.
>
Hmm...I've never used this, but in kmem_cache_alloc():
/*
* Enable redzoning and last user accounting, except for caches with
* large objects, if the increased size would increase the object size
* above the next power of two: caches with object sizes just above a
* power of two have a significant amount of internal fragmentation.
*/
if (size < 4096 || fls(size - 1) == fls(size-1 + REDZONE_ALIGN +
2 * sizeof(unsigned long long)))
flags |= SLAB_RED_ZONE | SLAB_STORE_USER;
...looks like it specifically excludes some caches.
> On Wed, Jun 11, 2008 at 02:46:13PM -0400, bfields wrote:
> > On Tue, Jun 10, 2008 at 05:12:31PM -0500, Weathers, Norman R. wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: J. Bruce Fields [mailto:bfields@xxxxxxxxxxxx]
> > > > Sent: Tuesday, June 10, 2008 12:16 PM
> > > > To: Weathers, Norman R.
> > > > Cc: linux-nfs@xxxxxxxxxxxxxxx
> > > > Subject: Re: Problems with large number of clients and reads
> > > >
> > > > On Tue, Jun 10, 2008 at 09:30:18AM -0500, Weathers, Norman R. wrote:
> > > > > Unfortunately, I cannot stop the clients (middle of long running
> > > > > jobs). I might be able to test this soon. If I have the number of
> > > > > threads high, yes I can reduce the number of threads and it
> > > > appears to
> > > > > lower some of the memory, but even with as little as three threads,
> > > > > the memory usage climbs very high, just not as high as if there are
> > > > > say 8 threads. When the memory usage climbs high, it can cause the
> > > > > box to not respond over the network (ssh, rsh), and even be very
> > > > > sluggish when I am connected over our serial console to the
> > > > server(s).
> > > > > This same scenario has been happening with kernels that I have tried
> > > > > from 2.6.22.x on to the 2.6.25 series. The 2.6.25 series is
> > > > > interesting in that I can push the same load from a box with the
> > > > > 2.6.25 kernel and not have a load over .3 (with 3 threads), but with
> > > > > the 2.6.22.x kernel, I have a load of over 3 when I hit the same
> > > > > conditions.
> > > >
> > > > OK, I think what we want to do is turn on
> > > > CONFIG_DEBUG_SLAB_LEAK. I've
> > > > never used it before, but it looks like it will report which functions
> > > > are allocating from each slab cache, which may be exactly what we need
> > > > to know. So:
> > > >
> > > > 1. Install a kernel with both CONFIG_DEBUG_SLAB ("Debug slab
> > > > memory allocations") and CONFIG_DEBUG_SLAB_LEAK ("Memory leak
> > > > debugging") turned on. They're both under the "kernel hacking"
> > > > section of the kernel config. (If you have a file
> > > > /proc/slab_allocators, then you already have these turned on and
> > > > you can skip this step.)
> > > >
> > > > 2. Do whatever you need to do to reproduce the problem.
> > > >
> > > > 3. Get a copy of /proc/slabinfo and /proc/slab_allocators.
> > > >
> > > > Then we can take a look at that and see if it sheds any light.
> > >
> > >
> > > I have taken several snapshots of the /proc/slab_allocators and
> > > /proc/slabinfo as requested, but since there is a lot of info in them,
> > > and I didn't think anyone wanted to go cross-eyed reading the data in an
> > > email, I have them up on a website:
> > >
> > > http://shashi-weathers.net/linux/cluster/NFS/
> >
> > Excellent.
> >
> > >
> > > The order of data collection is:
> > >
> > > slab_allocators_bad1.txt and corresponding slabinfo
> > > slab_allocators_after_bad1.txt and corresponding slabinfo
> > > slab_allocators_16_threads.txt and corresponding slabinfo
> > > slab_allocators_16_threads_1.txt and corresponding slabinfo
> > > slab_allocators_32_threads.txt and corresponding slabinfo
> > > slab_allocators_really_bad.txt and corresponding slabinfo.
> > >
> > >
> > > You will have to forgive my ignorance at this point, but I was looking
> > > through the slabinfo and slab_allocators, and noticed that size-4096
> > > does not show up in slab_allocators... I hope that is by design. You
> > > can see it growing into the gigabytes in the slabinfo files....
> >
> > Argh. OK, I don't understand well enough how this works. Time to ask
> > someone, I guess....
> >
> > --b.
> >
> > >
> > >
> > >
> > > >
> > > > I think that debugging will hurt the server performance, so you won't
> > > > want to keep it turned on all the time.
> > > >
> > > > >
> > > > > Also, this is all with the SLAB cache option. SLUB crashes
> > > > everytime
> > > > > I use it under heavy load.
> > > >
> > > > Have you reported the SLUB bugs to lkml?
> > >
> > > No, I haven't yet. I didn't know for sure if I was doing something
> > > wrong, or if SLUB was the problem there. Since the failures, I had gone
> > > back to using SLAB anyway, so .... I probably should...
> > >
> > > >
> > > > --b.
> > > >
> > >
> > >
> > > Norman Weathers
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--
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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