Re: Problems with large number of clients and reads

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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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