Messages in this thread | | | Date | Mon, 6 Jun 2022 04:54:24 +0100 | Subject | Re: [PATCH v4 3/3] squashfs: implement readahead | From | Phillip Lougher <> |
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On 03/06/2022 16:58, Marek Szyprowski wrote: > Hi Matthew, > > On 03.06.2022 17:29, Matthew Wilcox wrote: >> On Fri, Jun 03, 2022 at 10:55:01PM +0800, Hsin-Yi Wang wrote: >>> On Fri, Jun 3, 2022 at 10:10 PM Marek Szyprowski >>> <m.szyprowski@samsung.com> wrote: >>>> Hi Matthew, >>>> >>>> On 03.06.2022 14:59, Matthew Wilcox wrote: >>>>> On Fri, Jun 03, 2022 at 02:54:21PM +0200, Marek Szyprowski wrote: >>>>>> On 01.06.2022 12:39, Hsin-Yi Wang wrote: >>>>>>> Implement readahead callback for squashfs. It will read datablocks >>>>>>> which cover pages in readahead request. For a few cases it will >>>>>>> not mark page as uptodate, including: >>>>>>> - file end is 0. >>>>>>> - zero filled blocks. >>>>>>> - current batch of pages isn't in the same datablock or not enough in a >>>>>>> datablock. >>>>>>> - decompressor error. >>>>>>> Otherwise pages will be marked as uptodate. The unhandled pages will be >>>>>>> updated by readpage later. >>>>>>> >>>>>>> Suggested-by: Matthew Wilcox <willy@infradead.org> >>>>>>> Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org> >>>>>>> Reported-by: Matthew Wilcox <willy@infradead.org> >>>>>>> Reported-by: Phillip Lougher <phillip@squashfs.org.uk> >>>>>>> Reported-by: Xiongwei Song <Xiongwei.Song@windriver.com> >>>>>>> --- >>>>>> This patch landed recently in linux-next as commit 95f7a26191de >>>>>> ("squashfs: implement readahead"). I've noticed that it causes serious >>>>>> issues on my test systems (various ARM 32bit and 64bit based boards). >>>>>> The easiest way to observe is udev timeout 'waiting for /dev to be fully >>>>>> populated' and prolonged booting time. I'm using squashfs for deploying >>>>>> kernel modules via initrd. Reverting aeefca9dfae7 & 95f7a26191deon on >>>>>> top of the next-20220603 fixes the issue. >>>>> How large are these files? Just a few kilobytes? >>>> Yes, they are small, most of them are smaller than 16KB, some about >>>> 128KB and a few about 256KB. I've sent a detailed list in private mail. >>>> >>> Hi Marek, >>> >>> Are there any obvious squashfs errors in dmesg? Did you enable >>> CONFIG_SQUASHFS_FILE_DIRECT or CONFIG_SQUASHFS_FILE_CACHE? >> I don't think it's an error problem. I think it's a short file problem. >> >> As I understand the current code (and apologies for not keeping up >> to date with how the patch is progressing), if the file is less than >> msblk->block_size bytes, we'll leave all the pages as !uptodate, leaving >> them to be brough uptodate by squashfs_read_folio(). So Marek is hitting >> the worst case scenario where we re-read the entire block for each page >> in it. I think we have to handle this tail case in ->readahead(). > > I'm not sure if this is related to reading of small files. There are > only 50 modules being loaded from squashfs volume. I did a quick test of > reading the files. > > Simple file read with this patch: > > root@target:~# time find /initrd/ -type f | while read f; do cat $f > >/dev/null; done > > real 0m5.865s > user 0m2.362s > sys 0m3.844s > > Without: > > root@target:~# time find /initrd/ -type f | while read f; do cat $f > >/dev/null; done > > real 0m6.619s > user 0m2.112s > sys 0m4.827s >
It has been a four day holiday in the UK (Queen's Platinum Jubilee), hence the delay in responding.
The above read use-case is sequential (only one thread/process), whereas the use-case where the slow-down is observed may be parallel (multiple threads/processes entering Squashfs).
The above sequential use-case if the small files are held in fragments, will be exhibiting caching behaviour that will ameliorate the case where the same block is being repeatedly re-read for each page in it. Because each time Squashfs is re-entered handling only a single page, the decompressed block will be found in the fragment cache, eliminating a block decompression for each page.
In a parallel use-case the decompressed fragment block may be being eliminated from the cache (by other reading processes), hence forcing the block to be repeatedly decompressed.
Hence the slow-down will be much more noticable with a parallel use-case than a sequential use-case. It also may be why this slipped through testing, if the test cases are purely sequential in nature.
So Matthew's previous comment is still the most likely explanation for the slow-down.
Phillip
> Best regards
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