Con Kolivas wrote:
On Tue, 7 Feb 2006 04:00 pm, Nick Piggin wrote:
Con Kolivas wrote:
On Tue, 7 Feb 2006 02:08 pm, Nick Piggin wrote:
prefetch_get_page is doing funny things with zones and nodes / zonelists
(eg. 'We don't prefetch into DMA' meaning something like 'this only works
on i386 and x86-64').
Hrm? It's just a generic thing to do; I'm not sure I follow why it's i386
and x86-64 only. Every architecture has ZONE_NORMAL so it will prefetch
there.
I don't think every architecture has ZONE_NORMAL.
!ZONE_DMA they all have, no?
Don't think so. IIRC ppc64 has only ZONE_DMA although may have picked up
DMA32 now (/me boots the G5). IA64 I think have 4GB ZONE_DMA so smaller
systems won't have any other zones.
On small memory systems, ZONE_DMA will be a significant portion of memory
too (but maybe you're not targetting them either).
If you omit __GFP_WAIT and already test the watermarks yourself it should
be OK.
Ok.
Note, it may dip lower than we would like, but the watermark checking is
already completely racy anyway so it is possible that that will happen
anyway.
Workstations can have 2 or more dual core CPUs with multiple threads or
NUMA these days. Desktops and laptops will probably eventually gain more
cores and threads too.
While I am aware of the hardware changes out there I still doubt the
scalability issues you're concerned about affect a desktop. The code cost and
complexity will increase substantially yet I'm not sure that will be for any
gain to the targetted users.
Possibly. Why wouldn't you want swap prefetching on servers though?
Especially on some kind of shell server, or other internet server
where load could be really varied.
Why bother with the trylocks? On many architectures they'll RMW the
cacheline anyway, so scalability isn't going to be much improved (or do
you see big lock contention?)
Rather than scalability concerns per se the trylock is used as yet
another (admittedly rarely hit) way of defining busy.
They just seem to complicate the code for apparently little gain.
No biggie; I'll drop them.
That's what I'd do for now. A concurrent spin_lock could hit right after
the trylock takes the lock anyway...
The code is pretty aggressive at defining busy. It looks for pretty much
all of those and it prefetches till it stops then allowing idle to occur
again. Opting out of prefetching whenever there is doubt seems reasonable
to me.
What if you want to prefetch when there is slight activity going on though?
I don't. I want this to not cost us anything during any activity.
So if you have say some networking running (p2p or something), then it
may not ever prefetch?
What if your pagecache has filled memory with useless stuff (which would
appear to be the case with updatedb).
There is no way the vm will ever be smart enough to say "this is crap, throw
it out and prefetch some good stuff", so it doesn't matter.
It can do a lot better about throwing out updatedb type stuff.
Actually I had thought the point of this was to page in stuff after the
updatedb run, but it would appear that it won't do this because updatedb
will leave the pagecache full...
- for all its efforts, it will still interact with page reclaim by
putting pages on the LRU and causing them to be cycled.
- on bursty loads, this cycling could happen a bit. and more reads on
the swap devices.
Theoretically yes I agree. The definition of busy is so broad that
prevents it prefetching that it is not significant.
Not if the workload is very bursty.
It's an either/or for prefetching; I don't see a workaround, just some sane
balance.
Makes improving the rest of the VM for desktop users harder, no matter
how sane. Though I can't deny it is potentially an improvement itself
either.
Any code in a core system is intrusive by definition because it simply
adds to the amount of work that needs to be done when maintaining the
thing or trying to understand how things work, debugging people's badly
behaving workloads, etc.
I'm open to code suggestions and appreciate any outside help.
Hopefully you have a bit to go on. I still see difficult problems that
I'm not sure how can be solved.
If it is going to be off by default, why couldn't they
echo 10 > /proc/sys/vm/swappiness rather than turning it on?
Because we still swap no matter what the sysctl setting is, which makes it
even more useful in my opinion for those who aggressively set this tunable.
Sounds like we need to do more basic VM tuning as well.
--
SUSE Labs, Novell Inc.
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