Messages in this thread |  | | | Date | Wed, 31 May 2006 10:32:58 +1000 | | From | Nick Piggin <> | | Subject | Re: [rfc][patch] remove racy sync_page? |
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Linus Torvalds wrote: > > On Tue, 30 May 2006, Nick Piggin wrote: > >>For workloads where plugging helps (ie. lots of smaller, contiguous >>requests going into the IO layer), the request pattern should be >>pretty good without plugging these days, due to multiple page >>readahead and writeback. > > > No. > > That's fundamentally wrong. > > The fact is, plugging is not about read-ahead and writeback. It's very > fundamentally about the _boundaries_ between multiple requests, and in > particular the time when the queue starts out empty so that we can build > up things for devices that wand big requests, but even more so for devices > where _seeking_ is very expensive. > > Those boundaries haven't gone anywhere. The fact that we do read-ahead and > write-back in chunks doesn't change anything: yes, we often have the "big > requests" thing handled, but (a) not always and (b) upper layers > fundamentally don't fix the seek issues.
The requests can only get merged if contiguous requests from the upper layers come down, right?
So in a random IO workload, plugging is unlikely to help at all. In a contiguous IO workload, mpage should take *some* of the burden off plugging. But OK, it turns out not always, I accept that.
> > I want to know that the block layer could - if we wanted to - do things > like read-ahead for many distinct files, and for metadata. We don't > currently do much of that yet, but the point is, plugging _allows_ us to. > Exactly because it doesn't depend on upper layers feeding everything in > one go. > > Look at "sys_readahead()", and realize that it can be used to start IO for > read ahead _across_many_small_files_. Last I tried it, it was hugely > faster at populating the page cache than reading individual files (I used > to do it with BK to bring everything into cache so that the regular ops > would be fster - now git doesn't much need it). > > And maybe it was just my imagination, but the disk seemed quieter too. It > should be able to do better seek patterns at the beginning due to plugging > (ie we won't start IO after the first file, but after the request queue > fills up or something else needs to wait and we do an unplug event). > > THAT is what plugging is good for. Our read-ahead does well for large > requests, and that's important for some disk controllers in particular. > But plugging is about avoiding startign the IO too early.
Why would plugging help if the requests can't get merged, though?
> > Think about the TCP plugging (which is actually newer, but perhaps easier > to explain): it's useful not for the big file case (just use large reads > and writes), but for the "different sources" case - for handling the gap > between a header and the actual file contents. Exactly because it plugs in > _between_ events.
TCP plugging is a bit different because there is no page cache between the application and the device; and it is stream based so everything can be merged (within a single socket).
The same high level concept I agree, but I never said the concept was wrong; just hoped that as a heuristic, the block plugging was no longer useful. I've been set straight about that though ;)
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