* Avi Kivity <[email protected]> wrote:
> On 05/30/2011 05:10 PM, Ingo Molnar wrote:
> >> [...] Windows has a zeroing thread which causes all of RAM to be
> >> committed shortly after boot, though.
> >
> > heh, maybe they read lkml and copied my ancient idea:
> >
> > http://people.redhat.com/mingo/clearpage-patches/clearpage-2.3.18-J1
> >
> > An earlier version had a 'zerod' (page zeroing kernel thread).
> >
> > This was one of my more stupid ideas btw.
> >
>
> I think that with a dma engine it makes sense. We've got an extra
> resource, why not utilize it in the background? Some workloads
> generate a lot of demand for zero pages.
It was one of the early ideas to use DMA engines to clear memory
slowly in the background.
> I agree that using the cpu to clear memory is not a good idea, it
> just causes cache pollution.
Yeah, but even cache-neutral clearing (either driven from the CPU
from the idle thread or by a DMA engine) is not a particularly good
idea: because it uses up a finite resource: memory bandwidth. Can we
create 'idle' DMA transactions - once that never get in the way of
real DMA transactions?
Also, a profile of a typical kernel build shows:
0.69% cc1 [kernel.kallsyms] [k] clear_page_c
0.49% cc1 [kernel.kallsyms] [k] page_fault
So while we could improve it, the question is, can we do this without
accidentally slowing things down by more than 0.69%? And kernel
builds are a pretty clear_page_c() intense workload.
Thanks,
Ingo
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