linux-kernel - Re: [PATCH 0/3] THP Shrinker

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Message-ID: <c4462af8-80a5-6cb6-8bfc-0e7ff1da6856@redhat.com>
Date:   Wed, 28 Sep 2022 16:21:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     alexlzhu@...com, linux-mm@...ck.org
Cc:     willy@...radead.org, akpm@...ux-foundation.org, riel@...riel.com,
        hannes@...xchg.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 0/3] THP Shrinker

On 28.09.22 08:44, alexlzhu@...com wrote:
> From: Alexander Zhu <alexlzhu@...com>
> 
> Transparent Hugepages use a larger page size of 2MB in comparison to
> normal sized pages that are 4kb. A larger page size allows for fewer TLB
> cache misses and thus more efficient use of the CPU. Using a larger page
> size also results in more memory waste, which can hurt performance in some
> use cases. THPs are currently enabled in the Linux Kernel by applications
> in limited virtual address ranges via the madvise system call.  The THP
> shrinker tries to find a balance between increased use of THPs, and
> increased use of memory. It shrinks the size of memory by removing the
> underutilized THPs that are identified by the thp_utilization scanner.
> 
> In our experiments we have noticed that the least utilized THPs are almost
> entirely unutilized.
> 
> Sample Output:
> 
> Utilized[0-50]: 1331 680884
> Utilized[51-101]: 9 3983
> Utilized[102-152]: 3 1187
> Utilized[153-203]: 0 0
> Utilized[204-255]: 2 539
> Utilized[256-306]: 5 1135
> Utilized[307-357]: 1 192
> Utilized[358-408]: 0 0
> Utilized[409-459]: 1 57
> Utilized[460-512]: 400 13
> Last Scan Time: 223.98s
> Last Scan Duration: 70.65s
> 
> Above is a sample obtained from one of our test machines when THP is always
> enabled. Of the 1331 THPs in this thp_utilization sample that have from
> 0-50 utilized subpages, we see that there are 680884 free pages. This
> comes out to 680884 / (512 * 1331) = 99.91% zero pages in the least
> utilized bucket. This represents 680884 * 4KB = 2.7GB memory waste.
> 
> Also note that the vast majority of pages are either in the least utilized
> [0-50] or most utilized [460-512] buckets. The least utilized THPs are
> responsible for almost all of the memory waste when THP is always
> enabled. Thus by clearing out THPs in the lowest utilization bucket
> we extract most of the improvement in CPU efficiency. We have seen
> similar results on our production hosts.
> 
> This patchset introduces the THP shrinker we have developed to identify
> and split the least utilized THPs. It includes the thp_utilization
> changes that groups anonymous THPs into buckets, the split_huge_page()
> changes that identify and zap zero 4KB pages within THPs and the shrinker
> changes. It should be noted that the split_huge_page() changes are based
> off previous work done by Yu Zhao.
> 
> In the future, we intend to allow additional tuning to the shrinker
> based on workload depending on CPU/IO/Memory pressure and the
> amount of anonymous memory. The long term goal is to eventually always
> enable THP for all applications and deprecate madvise entirely.
> 
> In production we thus far have observed 2-3% reduction in overall cpu
> usage on stateless web servers when THP is always enabled.

What's the diff to the RFC?

-- 
Thanks,

David / dhildenb