Messages in this thread | | | Date | Thu, 24 Oct 2019 13:41:15 +0800 | From | Gao Xiang <> | Subject | Re: [PATCH] squashfs: Migrate from ll_rw_block usage to BIO |
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On Thu, Oct 24, 2019 at 10:23:54AM +0900, Philippe Liard wrote: > Thanks Cristoph for taking a look. I like the idea of simplifying this if > possible. I think I understand your suggestion in principle but I'm not > seeing a way to apply it here. Would it be possible for you to be a little > more specific? Let me try to explain this below. > > My admittedly?limited understanding is that using BIO indirectly requires > buffer_head or an alternative including some synchronization mechanism at > least. > It's true that the bio_{alloc,add_page,submit}() functions don't require > passing a buffer_head. However because bio_submit() is asynchronous AFAICT > the client needs to use a synchronization mechanism to wait for and notify > the completion of the request which buffer heads provide. This is achieved > respectively by wait_on_buffer() and {set,clear}_buffer_uptodate(). > > Another dependency on buffer heads is the fact that squashfs_read_data() > calls into other squashfs functions operating on buffer heads outside this > file. For example squashfs_decompress() operates on a buffer_head array. > > Given that bio_submit() is asynchronous I'm also not seeing how the > squashfs_bio_request allocation can be removed? There can be multiple BIO > requests in flight each needing to carry some context used on completion > of the request.
Personally speaking, just for Android related use cases, I'd suggest latest EROFS if you care more about system overall performance more than compression ratio, even https://lkml.org/lkml/2017/9/22/814 is applied (you can do benchmark), we did much efforts 3 years ago.
And that is not only performance but noticable memory overhead (a lot of extra memory allocations) and heavy page cache thrashing in low memory scenarios (it's very common [1].)
[1] https://linuxplumbersconf.org/event/4/contributions/404/attachments/326/550/Handling_memory_pressure_on_Android.pdf
Thanks, Gao Xiang
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