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Message-ID: <CAOvWMLbZF41QrY32AxMPRAJ2mSTKP3xMNZA=q7P5zj_saMNhag@mail.gmail.com> Date: Thu, 7 Nov 2013 14:45:31 -0800 From: Andiry Xu <andiry@...il.com> To: Jan Kara <jack@...e.cz> Cc: Wang Shilong <wangsl-fnst@...fujitsu.com>, linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org, Andiry Xu <andiry.xu@...il.com> Subject: Re: [BUG][ext2] XIP does not work on ext2 On Thu, Nov 7, 2013 at 2:20 PM, Jan Kara <jack@...e.cz> wrote: > On Thu 07-11-13 13:50:09, Andiry Xu wrote: >> On Thu, Nov 7, 2013 at 1:07 PM, Jan Kara <jack@...e.cz> wrote: >> > On Thu 07-11-13 12:14:13, Andiry Xu wrote: >> >> On Wed, Nov 6, 2013 at 1:18 PM, Jan Kara <jack@...e.cz> wrote: >> >> > On Tue 05-11-13 17:28:35, Andiry Xu wrote: >> >> >> >> Do you know the reason why write() outperforms mmap() in some cases? I >> >> >> >> know it's not related the thread but I really appreciate if you can >> >> >> >> answer my question. >> >> >> > Well, I'm not completely sure. mmap()ed memory always works on page-by-page >> >> >> > basis - you first access the page, it gets faulted in and you can further >> >> >> > access it. So for small (sub page size) accesses this is a win because you >> >> >> > don't have an overhead of syscall and fs write path. For accesses larger >> >> >> > than page size the overhead of syscall and some initial checks is well >> >> >> > hidden by other things. I guess write() ends up being more efficient >> >> >> > because write path taken for each page is somewhat lighter than full page >> >> >> > fault. But you'd need to look into perf data to get some hard numbers on >> >> >> > where the time is spent. >> >> >> > >> >> >> >> >> >> Thanks for the reply. However I have filled up the whole RAM disk >> >> >> before doing the test, i.e. asked the brd driver to allocate all the >> >> >> pages initially. >> >> > Well, pages in ramdisk are always present, that's not an issue. But you >> >> > will get a page fault to map a particular physical page in process' >> >> > virtual address space when you first access that virtual address in the >> >> > mapping from the process. The cost of setting up this virtual->physical >> >> > mapping is what I'm talking about. >> >> > >> >> >> >> Yes, you are right, there are page faults observed with perf. I >> >> misunderstood page fault as copying pages between backing store and >> >> physical memory. >> >> >> >> > If you had a process which first mmaps the file and writes to all pages in >> >> > the mapping and *then* measure the cost of another round of writing to the >> >> > mapping, I would expect you should see speeds close to those of memory bus. >> >> > >> >> >> >> I've tried this as well. mmap() performance improves but still not as >> >> good as write(). >> >> I used the perf report to compare write() and mmap() applications. For >> >> write() version, top of perf report shows as: >> >> 33.33% __copy_user_nocache >> >> 4.72% ext2_get_blocks >> >> 4.42% mutex_unlock >> >> 3.59% __find_get_block >> >> >> >> which looks reasonable. >> >> >> >> However, for mmap() version, the perf report looks strange: >> >> 94.98% libc-2.15.so [.] 0x000000000014698d >> >> 2.25% page_fault >> >> 0.18% handle_mm_fault >> >> >> >> I don't know what the first item is but it took the majority of cycles. >> > The first item means that it's some userspace code in libc. My guess >> > would be that it's libc's memcpy() function (or whatever you use to write >> > to mmap). How do you access the mmap? >> > >> >> Like this: >> >> fd = open(file_name, O_CREAT | O_RDWR | O_DIRECT, 0755); >> dest = (char *)mmap(NULL, FILE_SIZE, PROT_WRITE, MAP_SHARED, fd, 0); >> for (i = 0; i < count; i++) >> { >> memcpy(dest, src, request_size); >> dest += request_size; >> } > OK, maybe libc memcpy isn't very well optimized for you cpu? Not sure how > to tune that though... > Hmm, I will try some different kinds of memcpy to see if there is a difference. Just want to make sure I do not make some stupid mistakes before trying that. Thanks a lot for your help! Thanks, Andiry -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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