On 7 Sep 2020, at 3:20, Michal Hocko wrote:

> On Fri 04-09-20 14:10:45, Roman Gushchin wrote:
>> On Fri, Sep 04, 2020 at 09:42:07AM +0200, Michal Hocko wrote:
> [...]
>>> An explicit opt-in sounds much more appropriate to me as well. If we go
>>> with a specific API then I would not make it 1GB pages specific. Why
>>> cannot we have an explicit interface to "defragment" address space
>>> range into large pages and the kernel would use large pages where
>>> appropriate? Or is the additional copying prohibitively expensive?
>>
>> Can you, please, elaborate a bit more here? It seems like madvise(MADV_HUGEPAGE)
>> provides something similar to what you're describing, but there are lot
>> of details here, so I'm probably missing something.
>
> MADV_HUGEPAGE is controlling a preference for THP to be used for a
> particular address range. So it looks similar but the historical
> behavior is to control page faults as well and the behavior depends on
> the global setup.
>
> I've had in mind something much simpler. Effectively an API to invoke
> khugepaged (like) functionality synchronously from the calling context
> on the specific address range. It could be more aggressive than the
> regular khugepaged and create even 1G pages (or as large THPs as page
> tables can handle on the particular arch for that matter).
>
> As this would be an explicit call we do not have to be worried about
> the resulting latency because it would be an explicit call by the
> userspace.  The default khugepaged has a harder position there because
> has no understanding of the target address space and cannot make any
> cost/benefit evaluation so it has to be more conservative.

Something like MADV_HUGEPAGE_SYNC? It would be useful, since users have
better and clearer control of getting huge pages from the kernel and
know when they will pay the cost of getting the huge pages.

I would think the suggestion is more about the huge page control options
currently provided by the kernel do not have predictable performance
outcome, since MADV_HUGEPAGE is a best-effort option and does not tell
users whether the marked virtual address range is backed by huge pages
or not when the madvise returns. MADV_HUGEPAGE_SYNC would provide a
deterministic result to users on whether the huge page(s) are formed
or not.

—
Best Regards,
Yan Zi