|
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] MPI benchmark performance gap between native linux anddomU
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 commandecho -2 > /proc/sys/net/ipv4/tcp_adv_win_scale In my experiments, I notice the above changing doesn't persist upon reboots (every reboot will change the value back to 2, the default value for Debian Sarge 3.1). Is there a way to make a permanent changing? Thanks. Xuehai (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 zhangSent: Monday, April 04, 2005 4:19 PM To: Xen-devel@xxxxxxxxxxxxxxxxxxxSubject: [Xen-devel] MPI benchmark performance gap between native linux anddomUHi 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
|
 |
Lists.xenproject.org is hosted with RackSpace, monitoring our |