Re: objrmap-core-1 (rmap removal for file mappings to avoid 4:4 in<=16G machines)

[Andrew Morton]

Andrea Arcangeli <andrea@xxxxxxx> wrote:
>
> without this patch not even the 4:4 tlb overhead would allow intensive
> shm (shmfs+IPC) workloads to surivive on 32bit archs. Basically without
> this fix it's like 2.6 is running w/o pte-highmem.

yes.

> But the real reason of this work is for huge 64bit archs, so we speedup
> and avoid to waste tons of ram.

pte_chain space consumption is approximately equal to pagetable page space
consumption.  Sometimes a bit more, sometimes a lot less, approximately
equal.

So why do you say it saves "tons of ram"?

> on 32-ways the scalability is hurted
> very badly by rmap, so it has to be removed (Martin can provide the
> numbers I think).

I don't recall that the objrmap patches ever significantly affected CPU
utilisation.

I'm not saying that I'm averse to the patches, but I do suspect that this is
a case of large highmem boxes dragging the rest of the kernel along behind
them, and nothing else.
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