Messages in this thread | | | Date | Fri, 24 May 2002 03:04:37 -0700 | From | Andrew Morton <> | Subject | Re: Poor read performance when sequential write presents |
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William Lee Irwin III wrote: > > On Fri, May 24, 2002 at 02:26:48AM -0700, Andrew Morton wrote: > > Oh absolutely. That's the reason why 2.4 is beating 2.5 at tiobench with > > more than one thread. 2.5 is alternating fairly between threads and 2.4 > > is not. So 2.4 seeks less. > > In one sense or another some sort of graceful transition to unfair > behavior could be considered a kind of thrashing control; how meaningful > that is in the context of disk I/O is a question I can't answer directly, > though. Do you have any comments on this potential strategic unfairness?
Well we already have controls for strategic unfairness: the "read passovers" thing. It sets a finite limit on the number of times which a request can be walked past by the merging algorithm. Once that counter has expired, the request becomes effectively a merging barrier and it will propagate to the head of the queue as fast as the disk can retire the reads.
I don't have a problem if the `read latency' tunable (and this algorithm) cause a single thread to hog the disk head across multiple successive readahead windows. That's probably a good thing, and it's tunable.
But it seems to not be working right.
And there's no userspace tunable at this time.
> On Fri, May 24, 2002 at 02:26:48AM -0700, Andrew Morton wrote: > > I've been testing this extensively on 2.5 + multipage BIO I/O and when you > > increase readahead from 32 pages (two BIOs) to 64 pages (4 BIOs), 2.5 goes > > from perfect to horrid - each threads grabs the disk head and performs many, > > many megabytes of read before any other thread gets a share. Net effect is > > that the tiobench numbers are great, but any operation which involves > > reading disk has 30 or 60 second latencies. > > Interestingly, it seems specific to IDE. SCSI behaves well. > > I have tons of traces and debug code - I'll bug Jens about this in a week or > > so. > > What kinds of phenomena appear to be associated with IDE's latencies? > I recall some comments from prior IDE maintainers on poor interactions > between generic disk I/O layers and IDE drivers, particularly with > respect to small transactions being given to the drivers to perform. > Are these comments still relevant, or is this of a different nature?
I assume that there's a difference in the way in which the generic layer treats queueing for IDE devices. In 2.4, IDE devices are `head active', so the request at the head of the queue is under I/O. But SCSI isn't head-active. Requests get removed from the head of the queue prior to being serviced. At least, that's how I think it goes. I also believe that the 2.4 elevator does not look at the active request at the head when making merging decisions.
But in 2.5, head-activeness went away and as far as I know, IDE and SCSI are treated the same. Odd.
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