Re: active_mm and mm

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Tue, Aug 20, 2002 at 10:55:04AM -0400, Hai Huang wrote:
> Ok, I see why we're differentiating between mm and active_mm, but is this
> actually giving us a lot of benefits considering the number of context switches
> that would actually take advantage of this feature is probably small
> (well, it depends on the workload).

It's actually enormous.  There are a lot of kernel daemons that do
background IO, for example.  Those are often waking up after an IO
completes, doing a tiny amount of work to submit new IO, then sleeping
again.  Even more significant in many workloads is the idle task.

> Also, is the tlb flush operation that
> expensive?

Yes.  Modern cpus are _way_ faster than main memory, and they rely
utterly on the cache architecture to keep them busy.  Doing a tlb
flush forces the CPU to go back to main memory up to 2 times for every
single address translation that follows until the tlb is full again.
That's an enormous cost, especially on rapidly-switching workloads.

--Stephen
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