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Message-ID: <20131107222042.GC2054@quack.suse.cz> Date: Thu, 7 Nov 2013 23:20:42 +0100 From: Jan Kara <jack@...e.cz> To: Andiry Xu <andiry@...il.com> Cc: Jan Kara <jack@...e.cz>, 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 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... Honza -- Jan Kara <jack@...e.cz> SUSE Labs, CR -- 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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