> 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().
Thanks,
Song
>
>> What
>> would happen if the memory is so fragmented that we cannot allocate that
>> many huge pages? Do we need some fail over mechanisms?
>
> IIRC khugepaged preallocates pages without holding any locks and I would
> expect the same will be done for madvise as well.
> --
> Michal Hocko
> SUSE Labs
