[Devel] Re: Supporting overcommit with the memory controller

[Paul Menage]

On Wed, Mar 5, 2008 at 5:01 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu at jp.fujitsu.com> wrote:
>  > But to make this more interesting, there are plenty of jobs that will
>  > happily fill as much pagecache as they have available. Even a job
>  > that's just writing out logs will continually expand its pagecache
>  > usage without anything to stop it, and so just keeping the reserved
>  > pool at a fixed amount of free memory will result in the job expanding
>  > even if it doesn't need to.
>  It's current memory management style. "reclaim only when necessary".
>

Exactly - if the high-priority latency-sensitive job really needs that
extra memory, we want it to be able to automatically squash/kill the
low-priority job when memory runs low, and not suffer any latency
spikes. But if it doesn't actually need the memory, we'd rather use it
for low-priority batch stuff. The "no latency spikes" bit is important
- we don't want the high-priority job to get bogged down in
try_to_free_pages() and out_of_memory() loops when it needs to
allocate memory.

>  >
>  Can Balbir's soft-limit patches help ?
>
>  It reclamims each cgroup's pages to soft-limit if the system needs.
>
>  Make limitation  like this
>
>  Assume 4G server.
>                            Limit      soft-limit
>  Not important Apss:         2G          100M
>  Important Apps    :         3G          2.7G
>
>  When the system memory reachs to the limit, each cgroup's memory usages will
>  goes down to soft-limit. (And there will 1.3G of free pages in above example)
>

Yes, that could be a useful part of the solution - I suspect we'd need
to have kswapd do the soft-limit push back as well as in
try_to_free_pages(), to avoid the high-priority jobs getting stuck in
the reclaim code. It would also be nice if we had:

- a way to have the soft-limit pushing kick in substantially *before*
the machine ran out of memory, to provide a buffer for the
high-priority jobs.

- a way to measure the actual working set of a cgroup (which may be
smaller than its allocated memory if it has plenty of stale pagecache
pages allocated). Maybe refaults, or maybe usage-based information.

Paul
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