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xen-devel
RE: [Xen-devel] MPI benchmark performance gap between native linux anddo
Hi,
We had a similar network problem in the past. We were using a TCP
benchmark instead of MPI but I believe your problem is probably the same
as the one we encountered.
It took us a while to get to the bottom of this and we only identified
the reason for this behavior after we ported oprofile to Xen and did
some performance profiling experiments.
Here is a brief explanation of the problem we found and the solution
that worked for us.
Xenolinux allocates a full page (4KB) to store socket buffers instead
of using just MTU bytes as in traditional linux. This is necessary to
enable page exchanges between the guest and the I/O domains. The side
effect of this is that memory space used for socket buffers is not very
efficient. Even if packets have the maximum MTU size (typically 1500
bytes for Ethernet) the total buffer utilization is very low ( at most
just slightly higher than 35%). If packets arrive faster than they are
processed at the receiver side, they will exhaust the receiver buffer
before the TCP advertised window is reached (By default Linux uses a TCP
advertised window equal to 75% of the receive buffer size. In standard
Linux, this is typically sufficient to stop packet transmission at the
sender before running out of receive buffers. The same is not true in
Xen due to inefficient use of socket buffers). When a packet arrives and
there is no receive buffer available, TCP tries to free socket buffer
space by eliminating socket buffer fragmentation (i.e. eliminating
wasted buffer space). This is done at the cost of an extra copy of all
receive buffer to new compacted socket buffers. This introduces overhead
and reduces throughput when the CPU is the bottleneck, which seems to be
your case.
This problem is not very frequent because modern CPUs are fast enough to
receive packets at Gigabit speeds and the receive buffer does not fill
up. However the problem may arise when using slower machines and/or when
the workload consumes a lot of CPU cycles, such as for example
scientific MPI applications. In your case in you have both factors
against you.
The solution to this problem is trivial. You just have to change the TCP
advertised window of your guest to a lower value. In our case, we used
25% of the receive buffer size and that was sufficient to eliminate the
problem. This can be done using the following command
> echo -2 > /proc/sys/net/ipv4/tcp_adv_win_scale
(The default 2 corresponds to 75% of receive buffer, and -2 corresponds
to 25%)
Please let me know if this improve your results. You should still see a
degradation in throughput when comparing xen to traditional linux, but
hopefully you should be able to see better throughputs. You should also
try running your experiments in domain 0. This will give better
throughput although still lower than traditional linux.
I am curious to know if this have any effect in your experiments.
Please, post the new results if this has any effect in your results
Thanks
Renato
> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> xuehai zhang
> Sent: Monday, April 04, 2005 4:19 PM
> To: Xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] MPI benchmark performance gap between
> native linux anddomU
>
>
>
> Hi all,
>
> I did the following experiments to explore the MPI
> application execution performance on both native linux
> machines and inside of unpriviledged Xen user domains. I use
> 8 machines with identical HW configurations (498.756 MHz dual
> CPU, 512MB memory, on a 10MB/sec LAN) and I use Pallas MPI
> Benchmarks (PMB).
>
> Experiment 1: I boot all 8 nodes with native linux (nosmp,
> kernel 2.4.29) and use all of them for PMB tests.
>
> Experiment 2: I boot all 8 nodes with Xen running and start a
> single user domain (port 2.6.10,using file-backed VBD) on
> each node with 360MB memory. Then I run the same PMB tests
> among these 8 user domains.
>
> The expreiment results show, running a same MPI benchmark in
> user domains usually results in a worse (sometimes very bad)
> performance comparing with on native linux machines. The
> following are the results for PMB SendRecv benchmark for both
> experiments (table1 and table2 report throughput and latency
> respectively). As you may notice, SendRecv can achieve a
> 14.9MB/sec throughput on native linux machines but can get a
> maximum 7.07 MB/sec throughput if running inside of user
> domains. The latency results also have big gap.
>
> Clearly, there is difference between the memory used in the
> native linux machine of Experiment 1 (512MB) and in the user
> domain (360MB, can not go higher because dom0 started with
> 128MB memory) of Experiment 2. However, I don't think it is
> the main cause of the performance gap because the tested
> message sizes are much smaller than both memory sizes.
>
> I will appreciate your help if you had the similar experience
> and wanna share your insights.
>
> BTW, if you are not familar with PMB SendRecv benchmark, you
> can find a detailed explaination at
> http://people.cs.uchicago.edu/~hai/PMB-MPI1.pdf (see section 4.3.1).
>
> Thanks in advance for you help.
>
> Xuehai
>
>
> P.S. Table 1: SendRecv throughput (MB/sec) performance
>
> Message_Size(bytes) Experiment_1 Experiment_2
> 0 0 0
> 1 0 0
> 2 0 0
> 4 0 0
> 8 0.04 0.01
> 16 0.16 0.01
> 32 0.34 0.02
> 64 0.65 0.04
> 128 1.17 0.09
> 256 2.15 0.59
> 512 3.4 1.23
> 1K 5.29 2.57
> 2K 7.68 3.5
> 4K 10.7 4.96
> 8K 13.35 7.07
> 16K 14.9 3.77
> 32K 9.85 3.68
> 64K 5.06 3.02
> 128K 7.91 4.94
> 256K 7.85 5.25
> 512K 7.93 6.11
> 1M 7.85 6.5
> 2M 8.18 5.44
> 4M 7.55 4.93
>
> Table 2: SendRecv latency (millisec) performance
>
> Message_Size(bytes) Experiment_1 Experiment_2
> 0 1979.6 3010.96
> 1 1724.16 3218.88
> 2 1669.65 3185.3
> 4 1637.26 3055.67
> 8 406.77 2966.17
> 16 185.76 2777.89
> 32 181.06 2791.06
> 64 189.12 2940.82
> 128 210.51 2716.3
> 256 227.36 843.94
> 512 287.28 796.71
> 1K 368.72 758.19
> 2K 508.65 1144.24
> 4K 730.59 1612.66
> 8K 1170.22 2471.65
> 16K 2096.86 8300.18
> 32K 6340.45 17017.99
> 64K 24640.78 41264.5
> 128K 31709.09 50608.97
> 256K 63680.67 94918.13
> 512K 125531.7 162168.47
> 1M 251566.94 321451.02
> 2M 477431.32 707981
> 4M 997768.35 1503987.61
>
>
>
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