Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

Michal Hocko
2020-03-02T14:25:55Z

On Mon 02-03-20 22:12:53, Huang, Ying wrote:
> Michal Hocko <mhocko@kernel.org> writes:
[...]
> > And MADV_FREE pages are a kind of cache as well. If the target node is
> > short on memory then those will be reclaimed as a cache so a
> > pro-active freeing sounds counter productive as you do not have any
> > idea whether that cache is going to be used in future. In other words
> > you are not going to free a clean page cache if you want to use that
> > memory as a migration target right? So you should make a clear case
> > about why MADV_FREE cache is less important than the clean page cache
> > and ideally have a good justification backed by real workloads.
>
> Clean page cache still have valid contents, while clean MADV_FREE pages
> has no valid contents. So penalty of discarding the clean page cache is
> reading from disk, while the penalty of discarding clean MADV_FREE pages
> is just page allocation and zeroing.

And "just page allocation and zeroing" overhead is the primary
motivation to keep the page in memory. It is a decision of the workload
to use MADV_FREE because chances are that this will speed things up. All
that with a contract that the memory goes away under memory pressure so
with a good workload/memory sizing you do not really lose that
optimization. Now you want to make decision on behalf of the consumer of
the MADV_FREE memory.

> I understand that MADV_FREE is another kind of cache and has its value.
> But in the original implementation, during migration, we have already
> freed the original "cache", then reallocate the cache elsewhere and
> copy. This appears more like all pages are populated in mmap() always.
> I know there's value to populate all pages in mmap(), but does that need
> to be done always by default?

It is not. You have to explicitly request MAP_POPULATE to initialize
mmap.

--
Michal Hocko
SUSE Labs