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From: Herbert Poetzl (herbert_at_13thfloor.at)
Date: Wed 24 Nov 2004 - 13:02:07 GMT

On Wed, Nov 24, 2004 at 05:01:47PM +1300, Sam Vilain wrote:
> Jörn Engel wrote:
> >>...and the big challenge is - how do you apply this to memory usage?
> >Oh, you could. But the general concept behind my unquoted list is a
> >renewing resource. Network throughput is renewing. Network bandwidth
> >usually isn't. With swapping, you can turn memory into cache and
> >locality to the cpu is a renewable resource.
> Yep, that was my thought too. Memory seems like a static resource, so
> consider RSS used per second the renewable resource. Then you could
> charge tokens as normal.
> However, there are some tricky questions:
> 1) who do you charge shared memory (binaries etc) to ?
> 2) do you count mmap()'d regions in the buffercache?
> 3) if a process is sitting idle, but there is no VM contention, then
> they are "using" that memory more, so maybe they are using more
> "fast memory" tokens - but they might not really be occupying it,
> because it is not active.
> Maybe the thing with memory is that it's not important about how much is
> used per second, but more about how much active memory you are
> *displacing* per second into other places.
> We can find out from the VM subsystem how much RAM is displaced into
> swap by a context / process. It might also be possible for the MMU to

pages to be swapped out can not easily be assigned
to a context, this is different for pages getting
paged in ...

> report how much L2/L3 cache is displaced during a given slice. I have a
> hunch that the best solution to the memory usage problem will have to
> take into account the multi-tiered nature of memory. So, I think it
> would be excellent to be able to penalise contexts that thrash the L3
> cache. Systems with megabytes of L3 cache were designed to keep the
> most essential parts of most of the run queue hot - programs that thwart
> this by being bulky and excessively using pointers waste that cache.
> And then, it needs to all be done with no more than a few hundred cycles
> every reschedule. Hmm.
> Here's a thought about an algorithm that might work. This is all
> speculation without much regard to the existing implementations out
> there, of course. Season with grains of salt to taste.
> Each context is assigned a target RSS and VM size. Usage is counted a
> la disklimits (Herbert - is this already done?), but all complex

yep, not relative as with disklimits, but absolute
and in identical way the kernel accounts RSS and VM

> recalculation happens when somethings tries to swap something else out.
> As well as memory totals, each context also has a score that tracks how
> good or bad they've been with memory. Let's call that the "Jabba"
> value.
> When swap displacement occurs, it is first taken from disproportionately
> fat jabbas that are running on nearby CPUs (for NUMA). Displacing
> other's memory makes your context a fatter jabba too, but taking from
> jabbas that are already fat is not as bad as taking it from a hungry
> jabba. When someone takes your memory, that makes you a thinner jabba.
> This is not the same as simply a ratio of your context's memory usage to
> the allocated amount. Depending on the functions used to alter the
> jabba value, it should hopefully end up measuring something more akin to
> the amount of system memory turnover a context is inducing. It might
> also need something to act as a damper to pull a context's jabba nearer
> towards the zero point during lulls of VM activity.
> Then, if you are a fat jabba, maybe you might end up getting rescheduled
> instead of getting more memory whenever you want it!

thought about a simpler approach, with a TB for the
actual page-ins, so that every page-in will consume
a token, and you get a number per interval, as usual ...


> --
> Sam Vilain, sam /\T vilain |><>T net, PGP key ID: 0x05B52F13
> (include my PGP key ID in personal replies to avoid spam filtering)
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