Re: kswapd @ 60-80% CPU during heavy HD i/o.

[Rik van Riel <riel@conectiva.com.br>]

On Tue, 2 May 2000 frankeh@us.ibm.com wrote:

> It makes sense to me to make the number of pools configurable
> and not tie them directly to the number of nodes in a NUMA
> system. In particular allow memory pools (i.e. instance of
> pg_dat_t) to be smaller than a node size.

*nod*

We should have different memory zones per node on
Intel-handi^Wequipped NUMA machines.

> The smart things that I see has to happen is to allow a set of processes to
> be attached to a set of memory pools and the OS basically enforcing
> allocation in those constraints. I brought this up before and I think
> Andrea proposed something similar. Allocation should take place in those
> pools along the allocation levels based on GFP_MASK, so first allocate on
> HIGH along all specified pools and if unsuccessful, then fallback on a
> previous level.

That idea is broken if you don't do balancing of VM load between
zones.

> With each pool we should associate a kswapd.

How will local page replacement help you if the node next door
has practically unloaded virtual memory? You need to do global
page replacement of some sort...

> Making the size of the pools configurable allows to control the
> velocity at which we can swap out. Standard Queuing theory: if
> we can't get the desired througput, then increase the number of
> servers, here kswapd.

What we _could_ do is have one (or maybe even a few) kswapds
doing global replacement with io-less and more fine-grained
swap_out() and shrink_mmap() functions, and per-node kswapds
taking care of the IO and maybe even a per-node inactive list
(though that would probably be *bad* for page replacement).

Then again, if your machine can't get the desired throughput,
how would adding kswapds help??? Have you taken a look at
mm/page_alloc.c::alloc_pages()? If kswapd can't keep up, the
biggest memory consumers will help a hand and prevent the
rest of the system from thrashing too much.

regards,

Rik
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