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Date: Fri, 01 May 2009 16:05:38 -0400 From: George Wang <gwang@...espeedtech.com> To: linux-kernel@...r.kernel.org Subject: mmap() call blocks on Disk I/O? Hi, I am trouble shooting some disk I/O performance issues and have an interesting finding. The project is to improve event-driven web server performance in disk bound situation by avoiding blocking on disk I/O. We use separate threads to do disk I/O tasks, so we expect the main thread wont be badly affected by disk I/O, but it still is. I use strace to locate the sys-calls that block the main server thread when server is doing disk I/O intensive tasks. The finding is mmap() calls. we map files in 128KB memory blocks in the main thread. When disk I/O is low, mmap() usually takes 0.00002 second to complete, when the disk getting busy, it takes about 0.01 second to complete, if there are multiple disk I/O tasks making the disk seeks back and forth. mmap could take 0.1 second and up, the worst number we get is 1.4 seconds. Noticed that there is a patch to improve the multi-thread mmap() performance, it should not be the same issue as we have met, I believe. Just want to report this issue and hope it is helpful to improve the kernel. OpenSuse 11.0 with kernel 2.6.22.19-0.2-default, dual Intel Xeon 5410 (8-cores) with 8GB memory. Thanks, George Wang -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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