MPI benchmark performance gap between native linux and domU

[xuehai zhang <hai@cs.uchicago.edu>]

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