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.
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.
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.
Linus
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