On 7/23/07, Nick Piggin <[email protected]> wrote:
Ray Lee wrote:
> That said, I'm willing to run my day to day life through both a swap
> prefetch kernel and a normal one. *However*, before I go through all
> the work of instrumenting the damn thing, I'd really like Andrew (or
> Linus) to lay out his acceptance criteria on the feature. Exactly what
> *should* I be paying attention to? I've suggested keeping track of
> process swapin delay total time, and comparing with and without. Is
> that reasonable? Is it incomplete?
I don't feel it is so useful without more context. For example, in
most situations where pages get pushed to swap, there will *also* be
useful file backed pages being thrown out. Swap prefetch might
improve the total swapin delay time very significantly but that may
be just a tiny portion of the real problem.
Agreed, it's important to make sure we're not being penny-wise and
pound-foolish here.
Also a random day at the desktop, it is quite a broad scope and
pretty well impossible to analyse.
It is pretty broad, but that's also what swap prefetch is targetting.
As for hard to analyze, I'm not sure I agree. One can black-box test
this stuff with only a few controls. e.g., if I use the same apps each
day (mercurial, firefox, xorg, gcc), and the total I/O wait time
consistently goes down on a swap prefetch kernel (normalized by some
control statistic, such as application CPU time or total I/O, or
something), then that's a useful measurement.
If we can first try looking at
some specific problems that are easily identified.
Always easier, true. Let's start with "My mouse jerks around under
memory load." A Google Summer of Code student working on X.Org claims
that mlocking the mouse handling routines gives a smooth cursor under
load ([1]). It's surprising that the kernel would swap that out in the
first place.
[1] http://vignatti.wordpress.com/2007/07/06/xorg-input-thread-summary-or-something/
Looking at your past email, you have a 1GB desktop system and your
overnight updatedb run is causing stuff to get swapped out such that
swap prefetch makes it significantly better. This is really
intriguing to me, and I would hope we can start by making this
particular workload "not suck" without swap prefetch (and hopefully
make it even better than it currently is with swap prefetch because
we'll try not to evict useful file backed pages as well).
updatedb is an annoying case, because one would hope that there would
be a better way to deal with that highly specific workload. It's also
pretty stat dominant, which puts it roughly in the same category as a
git diff. (They differ in that updatedb does a lot of open()s and
getdents on directories, git merely does a ton of lstat()s instead.)
Anyway, my point is that I worry that tuning for an unusual and
infrequent workload (which updatedb certainly is), is the wrong way to
go.
After that we can look at other problems that swap prefetch helps
with, or think of some ways to measure your "whole day" scenario.
So when/if you have time, I can cook up a list of things to monitor
and possibly a patch to add some instrumentation over this updatedb
run.
That would be appreciated. Don't spend huge amounts of time on it,
okay? Point me the right direction, and we'll see how far I can run
with it.
Anyway, I realise swap prefetching has some situations where it will
fundamentally outperform even the page replacement oracle. This is
why I haven't asked for it to be dropped: it isn't a bad idea at all.
<nod>
However, if we can improve basic page reclaim where it is obviously
lacking, that is always preferable. eg: being a highly speculative
operation, swap prefetch is not great for power efficiency -- but we
still want laptop users to have a good experience as well, right?
Absolutely. Disk I/O is the enemy, and the best I/O is one you never
had to do in the first place.
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