linux-kernel - Re: [PATCH v3] mm: hugetlb: optionally allocate gigantic hugepages using cma 65;5803;1c Commit 944d9fec8d7a ("hugetlb: add support for gigantic page allocation at runtime") has added the run-time allocation of gigantic pages. However it actually works only at early stages of the system loading, when the majority of memory is free. After some time the memory gets fragmented by non-movable pages, so the chances to find a contiguous 1 GB block are getting close to zero. Even dropping caches manually doesn't help a lot.

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Date:   Wed, 1 Apr 2020 19:25:53 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Gushchin <guro@...com>
Cc:     Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Andreas Schaufler <andreas.schaufler@....de>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v3] mm: hugetlb: optionally allocate gigantic hugepages
 using cma 65;5803;1c Commit 944d9fec8d7a
 ("hugetlb: add support for gigantic page allocation at runtime") has added
 the run-time allocation of gigantic pages. However it actually works only
 at early stages of the system loading, when the majority of memory is free.
 After some time the memory gets fragmented by non-movable pages, so the
 chances to find a contiguous 1 GB block are getting close to zero. Even
 dropping caches manually doesn't help a lot.

On Wed, 11 Mar 2020 15:09:20 -0700 Roman Gushchin <guro@...com> wrote:

> At large scale rebooting servers in order to allocate gigantic hugepages
> is quite expensive and complex. At the same time keeping some constant
> percentage of memory in reserved hugepages even if the workload isn't
> using it is a big waste: not all workloads can benefit from using 1 GB
> pages.
> 
> The following solution can solve the problem:
> 1) On boot time a dedicated cma area* is reserved. The size is passed
>    as a kernel argument.
> 2) Run-time allocations of gigantic hugepages are performed using the
>    cma allocator and the dedicated cma area
> 
> In this case gigantic hugepages can be allocated successfully with a
> high probability, however the memory isn't completely wasted if nobody
> is using 1GB hugepages: it can be used for pagecache, anon memory,
> THPs, etc.
> 
> * On a multi-node machine a per-node cma area is allocated on each node.
>   Following gigantic hugetlb allocation are using the first available
>   numa node if the mask isn't specified by a user.
> 
> Usage:
> 1) configure the kernel to allocate a cma area for hugetlb allocations:
>    pass hugetlb_cma=10G as a kernel argument
> 
> 2) allocate hugetlb pages as usual, e.g.
>    echo 10 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> 
> If the option isn't enabled or the allocation of the cma area failed,
> the current behavior of the system is preserved.
> 
> x86 and arm-64 are covered by this patch, other architectures can be
> trivially added later.

Lots of review input on v2, but then everyone went quiet ;)

Has everything been addressed?