Re: high latency NFS

[Shehjar Tikoo]

J. Bruce Fields wrote:
> You might get more responses from the linux-nfs list (cc'd).
>
> --b.
>
> On Thu, Jul 24, 2008 at 01:11:31PM -0400, Michael Shuey wrote:
> > I'm currently toying with Linux's NFS, to see just how fast it 
> > can go in a high-latency environment.  Right now, I'm simulating
> >  a 100ms delay between client and server with netem (just 100ms
> > on the outbound packets from the client, rather than 50ms each
> > way). Oddly enough, I'm running into performance problems. :-)
> >
> > According to iozone, my server can sustain about 90/85 MB/s 
> > (reads/writes) without any latency added.  After a pile of 
> > tweaks, and injecting 100ms of netem latency, I'm getting 6/40 
> > MB/s (reads/writes).  I'd really like to know why writes are now
> >  so much faster than reads, and what sort of things might boost 
> > the read throughput.  Any suggestions?

Is the server sync or async mounted? I've seen such performance
inversion between read and write when the mount mode is async.

What is the number of nfsd threads at the server?

Which file system are you using at the server?

> > 1 The read throughput seems to be proportional to the latency - 
> > adding only 10ms of delay gives 61 MB/s reads, in limited testing
> >  (need to look at it further).  While that's to be expected, to 
> > some extent, I'm hoping there's some form of readahead that can 
> > help me out here (assume big sequential reads).
> >
> > iozone is reading/writing a file twice the size of memory on the
> >  client with a 32k block size.  I've tried raising this as high
> > as 16 MB, but I still see around 6 MB/sec reads.

In iozone, are you running the read and write test during the same run
of iozone? Iozone runs read tests, after writes so that the file for
the read test exists on the server. You should try running write and
read tests in separate runs to prevent client side caching issues from
influencing raw server read(and read-ahead) performance. You can use
the -w option in iozone to prevent iozone from calling unlink on the
file after the write test has finished, so you can use the same file
in a separate read test run.


> > I'm using a 2.6.9 derivative (yes, I'm a RHEL4 fan).  Testing 
> > with a stock 2.6, client and server, is the next order of 
> > business.

You can try building the kernel with oprofile support and use it to
measure where the client CPU is spending its time. It is possible that
client-side locking or other algorithm issues are resulting in such
low read throughput. Note, when you start oprofile profiling, use a
CPU_CYCLES count of 5000. I've observed more accurate results with
this sample size for NFS performance.

> > NFS mount is tcp, version 3.  rsize/wsize are 32k.  Both client 
> > and server have had tcp_rmem, tcp_wmem, wmem_max, rmem_max, 
> > wmem_default, and rmem_default tuned - tuning values are 12500000
> >  for defaults (and minimum window sizes), 25000000 for the 
> > maximums.  Inefficient, yes, but I'm not concerned with memory 
> > efficiency at the moment.
> >
> > Both client and server kernels have been modified to provide 
> > larger-than-normal RPC slot tables.  I allow a max of 1024, but 
> > I've found that actually enabling more than 490 entries in /proc
> >  causes mount to complain it can't allocate memory and die.  That
> >  was somewhat suprising, given I had 122 GB of free memory at the
> >  time...
> >
> > I've also applied a couple patches to allow the NFS readahead to
> >  be a tunable number of RPC slots.  Currently, I set this to 489
> >  on client and server (so it's one less than the max number of
> > RPC slots).  Bandwidth delay product math says 380ish slots
> > should be enough to keep a gigabit line full, so I suspect
> > something else is preventing me from seeing the readahead I
> > expect.
> >
> > FYI, client and server are connected via gigabit ethernet. 
> > There's a couple routers in the way, but they talk at 10gigE and
> >  can route wire speed. Traffic is IPv4, path MTU size is 9000 
> > bytes.

The following are not completely relevant here but just to get some
more info:
What is the raw TCP throughput that you get between the server and
client machine on this network?

You could run the tests with bare minimum number of network
elements between the server and the client to see whats the best
network performance for NFS you can extract from this server and
client machine.


> > Is there anything I'm missing?
> >
> > -- Mike Shuey Purdue University/ITaP
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