Messages in this thread | | | Date | Thu, 31 Jan 2019 12:34:44 +0100 (CET) | From | Jiri Kosina <> | Subject | Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O |
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On Thu, 31 Jan 2019, Florian Weimer wrote:
> >> I think this needs to use a different flag because the semantics are so > >> much different. If I understand this change correctly, previously, > >> RWF_NOWAIT essentially avoided any I/O, and now it does not. > > > > It still avoid synchronous I/O, due to this code still being in place: > > > > if (!PageUptodate(page)) { > > if (iocb->ki_flags & IOCB_NOWAIT) { > > put_page(page); > > goto would_block; > > } > > > > but goes the would_block path only after initiating asynchronous > > readahead. > > But it wouldn't schedule asynchronous readahead before?
It would, that's kind of the whole point.
> I'm worried that something, say PostgreSQL doing a sequential scan, > would implement a two-pass approach, first using RWF_NOWAIT to process > what's in the kernel page cache, and then read the rest without it. If > RWF_NOWAIT is treated as a prefetch hint, there could be much more read > activity, and a lot of it would be pointless because the data might have > to be evicted before userspace can use it.
So are you aware of anything already existing, that'd implement this semantics? I've quickly grepped https://github.com/postgres/postgres for RWF_NOWAIT, and they don't seem to use it at all. RWF_NOWAIT is rather new.
The usecase I am aware of is to make sure that the thread doing io_submit() doesn't get blocked for too long, because it has other things to do quickly in order to avoid starving other sub-threads (and delegate the I/O submission to asynchronous context).
-- Jiri Kosina SUSE Labs
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