Nick Piggin wrote:
Linus Torvalds wrote:
On Thu, 1 Jun 2006, Nick Piggin wrote:
If this wasn't clear: I don't mean per-task plugs as in "the task
explicitly plugs and unplugs the block device"[*]; I mean really
per-task plugs.
That would be insane. It would mean that you'd have to unplug whether you
I don't think it is.
wanted to or not. Ie you've now made "sys_readahead()" impossible to
do well, and doing read-ahead across multiple files.
Now it is you who are ignoring what I've been saying. I've been saying
that I don't think your sys_readahead examples have had much to do with
plugging:
1. If there are no other requests to seek to, plugging doesn't matter;
2. If there are other requests to seek to, the queue won't be plugged
or will soon become unplugged anyway. So the current system isn't
somehow going to do the right thing every time and be immune to
seeking.
Lastly, an app can still issue multiple sys_readaheads and still get
seek protection -- it isn't like disks have suddenly got faster than
CPUs overnight.
as-iosched.c can handle the case where a request goes out to disk, and
later a new one comes in nearby (deadline IIRC isn't good at this
unfortunately, nor does it seek backwards)
OTOH, if you're using as-iosched.c, you explicitly get a good amount of
seek protection anyway.
--
SUSE Labs, Novell Inc.
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