Re: Problems with large number of clients and reads

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Jun 03, 2008 at 01:50:01PM -0500, Norman Weathers wrote:
> Hello all,
> 
> We are having some issues with some high throughput servers of ours.
> 
> Here is the issue, we are using a vanilla 2.6.22.14 kernel on a node
> with 2 Dual Core Intels (3 GHz) and 16 GB of ram.  The files that are
> being served are around 2 GB each, and there are usually 3 to 5 of them
> being read, so once read they fit into memory nicely, and when all is
> working correctly, we have a perfectly filled cache, with almost no disk
> activity.
> 
> When we have large NFS activity (say, 600 to 1200 clients) connecting to
> the server(s), they can get into a state where they are using up all of
> memory, but they are dropping cache.  slabtop is showing 13 GB of memory
> being used by the size-4096 slab object.  We have two ethernet channels
> bonded, so we see in excess of 240 MB/s of data flowing out of the box,
> and all of the sudden, disk activity has risen to 185 MB/s.  This
> happens if we are using 8 or more nfs threads.  If we limit the threads
> to 6 or less, this doesn't happen.  Of course, we are starving clients,
> but at least the jobs that my customers are throwing out there are
> progressing.  The question becomes, what is causing the memory to be
> used up by the slab size-4096 object?  Why when all of the sudden a
> bunch of clients ask for data does this object grow from 100 MB to 13
> GB?  I have set the memory settings to something that I thought was
> reasonable.
> 
> Here is some more of the particulars:
> 
> sysctl.conf tcp memory settings:
> 
> # NFS Tuning Parameters
> sunrpc.udp_slot_table_entries = 128
> sunrpc.tcp_slot_table_entries = 128
> vm.overcommit_ratio = 80
> 
> net.core.rmem_max=524288
> net.core.rmem_default=262144
> net.core.wmem_max=524288
> net.core.wmem_default=262144
> net.ipv4.tcp_rmem = 8192 262144 524288
> net.ipv4.tcp_wmem = 8192 262144 524288
> net.ipv4.tcp_sack=0
> net.ipv4.tcp_timestamps=0
> vm.min_free_kbytes=50000
> vm.overcommit_memory=1
> net.ipv4.tcp_reordering=127
> 
> # Enable tcp_low_latency
> net.ipv4.tcp_low_latency=1
> 
> Here is a current reading from a slabtop of a system where this error is
> happening:
> 
> 3007154 3007154 100%    4.00K 3007154        1  12028616K size-4096
> 
> Note the size of the object cache, usually it is 50 - 100 MB (I have
> another box with 32 threads and the same settings which is bouncing
> between 50 and 128 MB right now).
> 
> I have a lot of client boxes that need access to these servers, and
> would really benefit from having more threads, but if I increase the
> number of threads, it pushes everything out of cache, forcing re-reads,
> and really slows down our jobs.
> 
> Any thoughts on this?

I'd've thought that suggests a leak of memory allocated by kmalloc().

Does the size-4096 cache decrease eventually, or does it stay that large
until you reboot?

--b.