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Date:   Tue, 5 Oct 2021 06:35:52 -0600
From:   Jens Axboe <>
To:     John Garry <>
Subject: Re: [PATCH v5 00/14] blk-mq: Reduce static requests memory footprint
 for shared sbitmap

On 10/5/21 4:23 AM, John Garry wrote:
> Currently a full set of static requests are allocated per hw queue per
> tagset when shared sbitmap is used.
> However, only tagset->queue_depth number of requests may be active at
> any given time. As such, only tagset->queue_depth number of static
> requests are required.
> The same goes for using an IO scheduler, which allocates a full set of
> static requests per hw queue per request queue.
> This series changes shared sbitmap support by using a shared tags per
> tagset and request queue. Ming suggested something along those lines in
> v1 review. In using a shared tags, the static rqs also become shared,
> reducing the number of sets of static rqs, reducing memory usage.
> Patch "blk-mq: Use shared tags for shared sbitmap support" is a bit big,
> and could potentially be broken down. But then maintaining ability to
> bisect becomes harder and each sub-patch would get more convoluted.
> For megaraid sas driver on my 128-CPU arm64 system with 1x SATA disk, we
> save approx. 300MB(!) [370MB -> 60MB]
> Baseline is 1b2d1439fc25 (block/for-next) Merge branch 'for-5.16/io_uring'
> into for-next

Let's get this queued up for testing, thanks John.

Jens Axboe

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