Re: MPI benchmark performance gap between native linux anddomU

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

Hi Ian and Jose,

Based on your suggestions, I did two more experiments: one (with tag "domU-B" in table below) is 
changing the TCP advertised window of domU to -2 (the default is 2) and the other (with tag "dom0" 
in table below) is to repeat the experiment in dom0 (only dom0 is running). The following table 
contains the results from these two new experiments plus two old ones (with tags "native-linux" and 
"domU-A" in table below) in my previous email.

I have the following observation from the results:

1. Decreasing the scaling of TCP window ("domU-B") doesn't buy any good to the  performance but 
slightly slowdown the performance (comparing with "domU-A").

2. Generally, the performance of running the experiments in dom0 ("dom0" column) is very close 
(slightly less) to the performance on native linux ("native-linux" column). However, in certain 
situations, it outperforms the performance on native linux. For example, throughput values when 
message size is 64KB and latency values when message size is 1 , or 2, or 4, or 8 bytes.

3. The performance gap between domU and dom0 is big, similarly as domU and native linux.

BTW, each reported data point in the following table is the average of over 10 runs of the same 
experiments. I forget to mention that in experiment using user domains, the 8 domU forms a private 
network and each domU is assigned a private network IP (for example, 192.168.254.X).

Xuehai

*********************************
*SendRecv Throughput(Mbytes/sec)*
*********************************

Msg Size(bytes)  native-linux	dom0          domU-A          domU-B
         0         0              0.00          0              0.00
         1         0              0.01          0              0.00
         2         0              0.01          0              0.00
         4         0              0.03          0              0.00
         8         0.04           0.05          0.01           0.01
        16         0.16           0.11          0.01           0.01
        32         0.34           0.21          0.02           0.02
        64         0.65           0.42          0.04           0.04
       128         1.17           0.79          0.09           0.10
       256         2.15           1.44          0.59           0.58
       512         3.4            2.39          1.23           1.22
      1024         5.29           3.79          2.57           2.50
      2048         7.68           5.30          3.5            3.44
      4096         10.7           8.51          4.96           5.23
      8192         13.35          11.06         7.07           6.00
     16384         14.9           13.60         3.77           4.62
     32768         9.85           11.13         3.68           4.34
     65536         5.06           9.06          3.02           3.14
    131072         7.91           7.61          4.94           5.04
    262144         7.85           7.65          5.25           5.29
    524288         7.93           7.77          6.11           5.40
   1048576         7.85           7.82          6.5            5.62
   2097152         8.18           7.35          5.44           5.32
   4194304         7.55           6.88          4.93           4.92

*********************************
*   SendRecv Latency(millisec)  *
*********************************

Msg Size(bytes)  native-linux	dom0           domU-A         domU-B					
         0         1979.6         1920.83       3010.96        3246.71    		
         1         1724.16        397.27        3218.88        3219.63
         2         1669.65        297.58        3185.3         3298.86
         4         1637.26        285.27        3055.67        3222.34
         8         406.77         282.78        2966.17        3001.24
        16         185.76         283.87        2777.89        2761.90
        32         181.06         284.75        2791.06        2798.77
        64         189.12         293.93        2940.82        3043.55
       128         210.51         310.47        2716.3         2495.83
       256         227.36         338.13        843.94         853.86
       512         287.28         408.14        796.71         805.51
      1024         368.72         515.59        758.19         786.67
      2048         508.65         737.12        1144.24        1150.66
      4096         730.59         917.97        1612.66        1516.35
      8192         1170.22        1411.94       2471.65        2650.17
     16384         2096.86        2297.19       8300.18        6857.13
     32768         6340.45        5619.56       17017.99       14392.36
     65536         24640.78       13787.31      41264.5        39871.19
    131072         31709.09       32797.52      50608.97       49533.68
    262144         63680.67       65174.67      94918.13       94157.30
    524288         125531.7       128116.73     162168.47      189307.05
   1048576         251566.94      252257.55     321451.02      361714.44
   2097152         477431.32      527432.60     707981         728504.38
   4194304         997768.35      1108898.61    1503987.61     1534795.56

Santos, Jose Renato G (Jose Renato Santos) wrote:
>   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@lists.xensource.com 
>>[mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of 
>>xuehai zhang
>>Sent: Monday, April 04, 2005 4:19 PM
>>To: Xen-devel@lists.xensource.com
>>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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>>
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