Messages in this thread | | | Date | Sun, 26 Aug 2001 14:05:37 -0600 | From | Victor Yodaiken <> | Subject | Re: [resent PATCH] Re: very slow parallel read performance |
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On Sun, Aug 26, 2001 at 04:38:55PM -0300, Rik van Riel wrote: > On Sun, 26 Aug 2001, Victor Yodaiken wrote: > > > And scheduling gets even more complex as we try to account for work > > done in this thread on behalf of other processes. And, of course, we > > have all sorts of wacky merge problems > > Actually, readahead is always done by the thread reading > the data, so this is not an issue.
Daniel was suggesting a readahead thread, if I'm not mistaken.
> > > BTW: maybe I'm oversimplifying, but since read-ahead is an optimization > > trading memory space for time, why doesn't it just turn off when there's > > a shortage of free memory? > > num_pages = (num_requestd_pages + (there_is_a_boatload_of_free_space? readahead: 0) > > When the VM load is high, the last thing you want to do is > shrink the size of your IO operations, this would only lead > to more disk seeks and possibly thrashing.
(obviously, I don't know anything about Linux VM, so my questions are ignorant)
Doesn't this very much depend on why VM load is high and on the kind of I/O load? For example, if your I/O load is already in big chunks or if VM stress is being caused by a bunch of big threads hammering shared data that is in page cache already. This is what I would guess happens in http servers where some more seeks might not even show up as a cost - or might be compensated for by read-ahead on the drive itself. And the OS should still be trying hard to aggregate swap I/O and I/O for paging. At least to me, "thrashing" where the OS is shuffling pages in and out without work getting done is different from "thrashing" where user processes run with suboptimal I/O. I think the applications people are better if they know If memory fills up, I/O may slow down because the OS won't do readaheads. than The OS will attempt to guess aggregate optimial I/O patterns and may get it completely wrong, so that when memory is full performance becomes totally unpredictable.
But since I have no numbers this is just a stab in the dark.
> It would be nice to do something similar to TCP window > collapse for readahead, though... > > This would work by increasing the readahead size every > time we reach the end of the last readahead window without > having to re-read data twice and collapsing the readahead > window if any of the pages we read in have to be read > twice before we got around to using them.
So suppose Dave Miller's computer does Process A: request data; sleep; Process B: fill memory with graphics (high res pictures of sparc 4s. of course) Process A wake up, readahead failed (pages seized by graphics); read more; sleep Process B fill memory Process A; get data; expand readahead; request readahead etc. That is, failure to use readahead may be caused by memory pressure, scheduling delays, etc - how do you tell the difference between a process that would profit from readahead if the scheduler would let it and one that would not?
> IA64: a worthy successor to i860.
Not the 432?
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