Re: [resent PATCH] Re: very slow parallel read performance

Victor Yodaiken
2001-08-26T20:10:00Z

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?