From: David Hildenbrand <david@redhat.com>
To: Balbir Singh <bsingharora@gmail.com>, Vlastimil Babka <vbabka@suse.cz>
Cc: Linux Memory Management List <linux-mm@kvack.org>,
Minchan Kim <minchan@kernel.org>,
Matthew Wilcox <willy@infradead.org>,
Rik van Riel <riel@surriel.com>, Michal Hocko <mhocko@kernel.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Peter Xu <peterx@redhat.com>
Subject: Re: Page zapping and page table reclaim
Date: Fri, 19 Mar 2021 13:44:55 +0100 [thread overview]
Message-ID: <6f6a0e7b-4b61-cab4-cf8b-7437d7149b21@redhat.com> (raw)
In-Reply-To: <20210318235347.GA3346@balbir-desktop>
On 19.03.21 00:53, Balbir Singh wrote:
> On Thu, Mar 18, 2021 at 05:57:06PM +0100, Vlastimil Babka wrote:
>> On 3/11/21 7:14 PM, David Hildenbrand wrote:
>>> Hi folks,
>>>
>>> I was wondering, is there any mechanism that reclaims basically empty page
>>> tables in a running process?
>>>
>>> Like: When I MADV_DONTNEED a huge range, there could be plenty of basically
>>> empty (e.g., all entries invalid) page tables we could reclaim. As soon as we
>>> zap a complete PMD we could reclaim (depending on the architecture) a whole page.
>>>
>>> Zapping on the PMD level might make most impact I guess.
>>>
>>> For 1 GB, we need 262144 4k pages. If we assume each PTE is 8 bytes, we need a
>>> total of 8 MB for the lowest level page tables (PTE).
>>>
>>> OTOH, we would need 512 PMD entries - a single 4k page. Zapping 1 TB would mean
>>> we can free up another 4MB - rather a corner case and we can live with that.
>>>
>>>
>>> Of course, the same might apply to other cases where we can restore all page
>>> table content from the VMA again. One example would be after MADV_FREE zapped a
>>> whole range of entries we marked.
>>
>> I don't think we have such mechanism, but IIRC I've heard the idea mentioned
>> before, probably from Michal Hocko. Definitely an interesting research project
>> idea to evaluate the cost vs benefits of that.
>>
>
> It might lead to interesting interactions with lockless page table walking
> with implications on the mmap_lock as well.
>
I think if lockless page table walks have to be able with THP code
swapping populated page tables by a PMD back and forth, swapping an
unpopulated page table by an invalid PMD entry might be quite similar.
At least it feels like both approaches would rely on similar mechanisms
/ locking. :)
I'm planning on looking into this, but not sure when I'll have time to
prototype something up.
> Balbir
>
>
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-03-19 12:45 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-11 18:14 Page zapping and page table reclaim David Hildenbrand
2021-03-11 21:26 ` Peter Xu
2021-03-11 21:35 ` David Hildenbrand
2021-03-19 17:04 ` Yang Shi
2021-03-22 9:34 ` David Hildenbrand
2021-03-18 16:57 ` Vlastimil Babka
2021-03-18 23:53 ` Balbir Singh
2021-03-19 12:44 ` David Hildenbrand [this message]
2021-03-20 1:56 ` Balbir Singh
2021-03-22 9:19 ` David Hildenbrand
2021-03-18 18:03 ` Rik van Riel
2021-03-18 18:15 ` David Hildenbrand
2021-03-24 9:55 ` David Hildenbrand
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