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Date:   Thu, 26 Oct 2023 14:18:43 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Peng Zhang <zhangpeng.00@...edance.com>
Cc:     corbet@....net, akpm@...ux-foundation.org, willy@...radead.org,
        brauner@...nel.org, surenb@...gle.com, michael.christie@...cle.com,
        mjguzik@...il.com, mathieu.desnoyers@...icios.com,
        npiggin@...il.com, peterz@...radead.org, oliver.sang@...el.com,
        mst@...hat.com, maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v6 00/10] Introduce __mt_dup() to improve the performance
 of fork()

* Peng Zhang <zhangpeng.00@...edance.com> [231024 04:33]:
> Hi all,
> 
> This series introduces __mt_dup() to improve the performance of fork(). During
> the duplication process of mmap, all VMAs are traversed and inserted one by one
> into the new maple tree, causing the maple tree to be rebalanced multiple times.
> Balancing the maple tree is a costly operation. To duplicate VMAs more
> efficiently, mtree_dup() and __mt_dup() are introduced for the maple tree. They
> can efficiently duplicate a maple tree.
> 
> Here are some algorithmic details about {mtree,__mt}_dup(). We perform a DFS
> pre-order traversal of all nodes in the source maple tree. During this process,
> we fully copy the nodes from the source tree to the new tree. This involves
> memory allocation, and when encountering a new node, if it is a non-leaf node,
> all its child nodes are allocated at once.
> 
> This idea was originally from Liam R. Howlett's Maple Tree Work email, and I
> added some of my own ideas to implement it. Some previous discussions can be
> found in [1]. For a more detailed analysis of the algorithm, please refer to the
> logs for patch [3/10] and patch [10/10].
> 
> There is a "spawn" in byte-unixbench[2], which can be used to test the
> performance of fork(). I modified it slightly to make it work with
> different number of VMAs.
> 
> Below are the test results. The first row shows the number of VMAs.
> The second and third rows show the number of fork() calls per ten seconds,
> corresponding to next-20231006 and the this patchset, respectively. The
> test results were obtained with CPU binding to avoid scheduler load
> balancing that could cause unstable results. There are still some
> fluctuations in the test results, but at least they are better than the
> original performance.
> 
> 21     121   221    421    821    1621   3221   6421   12821  25621  51221
> 112100 76261 54227  34035  20195  11112  6017   3161   1606   802    393
> 114558 83067 65008  45824  28751  16072  8922   4747   2436   1233   599
> 2.19%  8.92% 19.88% 34.64% 42.37% 44.64% 48.28% 50.17% 51.68% 53.74% 52.42%
> 
> Thanks to Liam and Matthew for the review.

Besides the one comment by Matthew, this series looks good.

> 
> Changes since v5:
>  - Correct the copyright statement.
>  - Add Suggested-by tag in patch [3/10] and [10/10], this work was originally
>    proposed by Liam R. Howlett.
>  - Some cleanup and comment corrections for patch [3/10].
>  - Use vma_iter* series interfaces as much as possible in [10/10].
> 
> [1] https://lore.kernel.org/lkml/463899aa-6cbd-f08e-0aca-077b0e4e4475@bytedance.com/
> [2] https://github.com/kdlucas/byte-unixbench/tree/master
> 
> v1: https://lore.kernel.org/lkml/20230726080916.17454-1-zhangpeng.00@bytedance.com/
> v2: https://lore.kernel.org/lkml/20230830125654.21257-1-zhangpeng.00@bytedance.com/
> v3: https://lore.kernel.org/lkml/20230925035617.84767-1-zhangpeng.00@bytedance.com/
> v4: https://lore.kernel.org/lkml/20231009090320.64565-1-zhangpeng.00@bytedance.com/
> v5: https://lore.kernel.org/lkml/20231016032226.59199-1-zhangpeng.00@bytedance.com/
> 
> Peng Zhang (10):
>   maple_tree: Add mt_free_one() and mt_attr() helpers
>   maple_tree: Introduce {mtree,mas}_lock_nested()
>   maple_tree: Introduce interfaces __mt_dup() and mtree_dup()
>   radix tree test suite: Align kmem_cache_alloc_bulk() with kernel
>     behavior.
>   maple_tree: Add test for mtree_dup()
>   maple_tree: Update the documentation of maple tree
>   maple_tree: Skip other tests when BENCH is enabled
>   maple_tree: Update check_forking() and bench_forking()
>   maple_tree: Preserve the tree attributes when destroying maple tree
>   fork: Use __mt_dup() to duplicate maple tree in dup_mmap()
> 

Besides 03/10, please add:
Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

Thanks for all the work on this,
Liam

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