On Thu 18-02-21 09:53:25, Song Liu wrote:
>
>
> > On Feb 18, 2021, at 12:39 AM, Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Thu 18-02-21 08:11:13, Song Liu wrote:
> >>
> >>
> >>> On Feb 16, 2021, at 8:24 PM, David Rientjes <rientjes@xxxxxxxxxx> wrote:
> >>>
> >>> Hi everybody,
> >>>
> >>> Khugepaged is slow by default, it scans at most 4096 pages every 10s.
> >>> That's normally fine as a system-wide setting, but some applications would
> >>> benefit from a more aggressive approach (as long as they are willing to
> >>> pay for it).
> >>>
> >>> Instead of adding priorities for eligible ranges of memory to khugepaged,
> >>> temporarily speeding khugepaged up for the whole system, or sharding its
> >>> work for memory belonging to a certain process, one approach would be to
> >>> allow userspace to induce hugepage collapse.
> >>>
> >>> The benefit to this approach would be that this is done in process context
> >>> so its cpu is charged to the process that is inducing the collapse.
> >>> Khugepaged is not involved.
> >>>
> >>> Idea was to allow userspace to induce hugepage collapse through the new
> >>> process_madvise() call. This allows us to collapse hugepages on behalf of
> >>> current or another process for a vectored set of ranges.
> >>>
> >>> This could be done through a new process_madvise() mode *or* it could be a
> >>> flag to MADV_HUGEPAGE since process_madvise() allows for a flag parameter
> >>> to be passed. For example, MADV_F_SYNC.
> >>>
> >>> When done, this madvise call would allocate a hugepage on the right node
> >>> and attempt to do the collapse in process context just as khugepaged would
> >>> otherwise do.
> >>
> >> This is very interesting idea. One question, IIUC, the user process will
> >> block until all small pages in given ranges are collapsed into THPs.
> >
> > Do you mean that PF would be blocked due to exclusive mmap_sem? Or is
> > there anything else oyu have in mind?
>
> I was thinking about memory defragmentation when the application asks for
> many THPs. Say the application looks like
>
> main()
> {
> malloc();
> madvise(HUGE);
> process_madvise();
>
> /* start doing work */
> }
>
> IIUC, when process_madvise() finishes, the THPs should be ready. However,
> if defragmentation takes a long time, the process will wait in process_madvise().
OK, I see. The operation is definitely free which is to be expected. You
can do the same from a thread which can spend time collapsing THPs.
There are still internal resources that might block others - e.g. the
above mentioned mmap_sem. We can try hard to reduce the lock time but
this is unlikely to be completely free of any interruption of the
workload.
--
Michal Hocko
SUSE Labs
