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From:   Song Liu <song@kernel.org>
Date:   Mon, 25 Apr 2022 16:07:13 -0700
Message-ID: <CAPhsuW6j7UYn-jH9-kmBfe5h+PJ_HjP9v6dW+VDKiMW+t+XUhA@mail.gmail.com>
Subject: Re: [PATCH v2 00/12] Improve Raid5 Lock Contention
To:     Logan Gunthorpe <logang@deltatee.com>
Cc:     open list <linux-kernel@vger.kernel.org>,
        linux-raid <linux-raid@vger.kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Guoqing Jiang <guoqing.jiang@linux.dev>,
        Stephen Bates <sbates@raithlin.com>,
        Martin Oliveira <Martin.Oliveira@eideticom.com>,
        David Sloan <David.Sloan@eideticom.com>
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On Wed, Apr 20, 2022 at 12:55 PM Logan Gunthorpe <logang@deltatee.com> wrote:
>
> Hi,
>
> This is v2 of this series which addresses Christoph's feedback and
> fixes some bugs. The first posting is at [1]. A git branch is
> available at [2].
>
> --
>
> I've been doing some work trying to improve the bulk write performance
> of raid5 on large systems with fast NVMe drives. The bottleneck appears
> largely to be lock contention on the hash_lock and device_lock. This
> series improves the situation slightly by addressing a couple of low
> hanging fruit ways to take the lock fewer times in the request path.
>
> Patch 9 adjusts how batching works by keeping a reference to the
> previous stripe_head in raid5_make_request(). Under most situtations,
> this removes the need to take the hash_lock in stripe_add_to_batch_list()
> which should reduce the number of times the lock is taken by a factor of
> about 2.
>
> Patch 12 pivots the way raid5_make_request() works. Before the patch, the
> code must find the stripe_head for every 4KB page in the request, so each
> stripe head must be found once for every data disk. The patch changes this
> so that all the data disks can be added to a stripe_head at once and the
> number of times the stripe_head must be found (and thus the number of
> times the hash_lock is taken) should be reduced by a factor roughly equal
> to the number of data disks.
>
> The remaining patches are just cleanup and prep patches for those two
> patches.
>
> Doing apples to apples testing this series on a small VM with 5 ram
> disks, I saw a bandwidth increase of roughly 14% and lock contentions
> on the hash_lock (as reported by lock stat) reduced by more than a factor
> of 5 (though it is still significantly contended).
>
> Testing on larger systems with NVMe drives saw similar small bandwidth
> increases from 3% to 20% depending on the parameters. Oddly small arrays
> had larger gains, likely due to them having lower starting bandwidths; I
> would have expected larger gains with larger arrays (seeing there
> should have been even fewer locks taken in raid5_make_request()).
>
> Logan
>
> [1] https://lkml.kernel.org/r/20220407164511.8472-1-logang@deltatee.com
> [2] https://github.com/sbates130272/linux-p2pmem raid5_lock_cont_v2
>

The set looks good to me overall. Thanks everyone for the review and feedback.

Logan, please incorporate feedback and send v3.

Thanks,
Song