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From: Mike Kravetz <mike.kravetz@oracle.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	David Hildenbrand <david@redhat.com>,
	Michal Hocko <mhocko@suse.com>,
	Oscar Salvador <osalvador@suse.de>, Zi Yan <ziy@nvidia.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Naoya Horiguchi <naoya.horiguchi@linux.dev>,
	David Rientjes <rientjes@google.com>,
	Vlastimil Babka <vbabka@suse.cz>, Hillf Danton <hdanton@sina.com>
Subject: Re: [PATCH RESEND 0/8] hugetlb: add demote/split page functionality
Date: Tue, 24 Aug 2021 15:08:46 -0700	[thread overview]
Message-ID: <10d86c18-f0cf-395f-4209-17ac71b9fc03@oracle.com> (raw)
In-Reply-To: <20210816184611.07b97f4c26b83090f5d48fab@linux-foundation.org>

Add Vlastimil and Hillf,

On 8/16/21 6:46 PM, Andrew Morton wrote:
> On Mon, 16 Aug 2021 17:46:58 -0700 Mike Kravetz <mike.kravetz@oracle.com> wrote:
> 
>>> It really is a ton of new code.  I think we're owed much more detail
>>> about the problem than the above.  To be confident that all this
>>> material is truly justified?
>>
>> The desired functionality for this specific use case is to simply
>> convert a 1G huegtlb page to 512 2MB hugetlb pages.  As mentioned
>>
>> "Converting larger to smaller hugetlb pages can be accomplished today by
>>  first freeing the larger page to the buddy allocator and then allocating
>>  the smaller pages.  However, there are two issues with this approach:
>>  1) This process can take quite some time, especially if allocation of
>>     the smaller pages is not immediate and requires migration/compaction.
>>  2) There is no guarantee that the total size of smaller pages allocated
>>     will match the size of the larger page which was freed.  This is
>>     because the area freed by the larger page could quickly be
>>     fragmented."
>>
>> These two issues have been experienced in practice.
> 
> Well the first issue is quantifiable.  What is "some time"?  If it's
> people trying to get a 5% speedup on a rare operation because hey,
> bugging the kernel developers doesn't cost me anything then perhaps we
> have better things to be doing.

Well, I set up a test environment on a larger system to get some
numbers.  My 'load' on the system was filling the page cache with
clean pages.  The thought is that these pages could easily be reclaimed.

When trying to get numbers I hit a hugetlb page allocation stall where
__alloc_pages(__GFP_RETRY_MAYFAIL, order 9) would stall forever (or at
least an hour).  It was very much like the symptoms addressed here:
https://lore.kernel.org/linux-mm/20190806014744.15446-1-mike.kravetz@oracle.com/

This was on 5.14.0-rc6-next-20210820.

I'll do some more digging as this appears to be some dark corner case of
reclaim and/or compaction.  The 'good news' is that I can reproduce
this.

> And the second problem would benefit from some words to help us
> understand how much real-world hurt this causes, and how frequently.
> And let's understand what the userspace workarounds look like, etc.

The stall above was from doing a simple 'free 1GB page' followed by
'allocate 512 MB pages' from userspace.

Getting out another version of this series will be delayed, as I think
we need to address or understand this issue first.
-- 
Mike Kravetz

  parent reply	other threads:[~2021-08-24 22:09 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-16 22:49 Mike Kravetz
2021-08-16 22:49 ` [PATCH 1/8] hugetlb: add demote hugetlb page sysfs interfaces Mike Kravetz
2021-08-16 22:49 ` [PATCH 2/8] hugetlb: add HPageCma flag and code to free non-gigantic pages in CMA Mike Kravetz
2021-08-16 22:49 ` [PATCH 3/8] hugetlb: add demote bool to gigantic page routines Mike Kravetz
2021-08-16 22:49 ` [PATCH 4/8] hugetlb: add hugetlb demote page support Mike Kravetz
2021-08-16 22:49 ` [PATCH 5/8] hugetlb: document the demote sysfs interfaces Mike Kravetz
2021-08-16 23:28   ` Andrew Morton
2021-08-17  1:04     ` Mike Kravetz
2021-09-21 13:52   ` Aneesh Kumar K.V
2021-09-21 17:17     ` Mike Kravetz
2021-08-16 22:49 ` [PATCH 6/8] hugetlb: vmemmap optimizations when demoting hugetlb pages Mike Kravetz
2021-08-16 22:49 ` [PATCH 7/8] hugetlb: prepare destroy and prep routines for vmemmap optimized pages Mike Kravetz
2021-08-16 22:49 ` [PATCH 8/8] hugetlb: Optimized demote vmemmap optimizatized pages Mike Kravetz
2021-08-16 23:23 ` [PATCH RESEND 0/8] hugetlb: add demote/split page functionality Andrew Morton
2021-08-17  0:17   ` Mike Kravetz
2021-08-17  0:39     ` Andrew Morton
2021-08-17  0:58       ` Mike Kravetz
2021-08-16 23:27 ` Andrew Morton
2021-08-17  0:46   ` Mike Kravetz
2021-08-17  1:46     ` Andrew Morton
2021-08-17  7:30       ` David Hildenbrand
2021-08-17 16:19         ` Mike Kravetz
2021-08-17 18:49           ` David Hildenbrand
2021-08-24 22:08       ` Mike Kravetz [this message]
2021-08-27 17:22         ` Vlastimil Babka
2021-08-27 23:04           ` Mike Kravetz
2021-08-30 10:11             ` Vlastimil Babka
2021-09-02 18:17               ` Mike Kravetz
2021-09-06 14:40                 ` Vlastimil Babka
     [not found]                 ` <20210907085001.3773-1-hdanton@sina.com>
2021-09-08 21:00                   ` Mike Kravetz
2021-09-09 11:54                     ` Michal Hocko
2021-09-09 13:45                       ` Vlastimil Babka
2021-09-09 21:31                         ` Mike Kravetz
2021-09-10  8:20                         ` Michal Hocko
2021-09-11  0:11                           ` Mike Kravetz
2021-09-13 15:50                             ` Michal Hocko
2021-09-15 16:57                               ` Mike Kravetz
2021-09-17 20:44                                 ` Mike Kravetz

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