linux-kernel - Re: [PATCH] nvme-pci: fix host memory buffer allocation size

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Message-ID: <20220428150644.GA22685@lst.de>
Date:   Thu, 28 Apr 2022 17:06:44 +0200
From:   Christoph Hellwig <hch@....de>
To:     Thomas Weißschuh <linux@...ssschuh.net>
Cc:     Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>,
        Jens Axboe <axboe@...com>, Sagi Grimberg <sagi@...mberg.me>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH] nvme-pci: fix host memory buffer allocation size

On Thu, Apr 28, 2022 at 04:44:47PM +0200, Thomas Weißschuh wrote:
> Is the current code supposed to reach HMPRE? It does not for me.
> 
> The code tries to allocate memory for HMPRE in chunks.
> The best allocation would be to allocate one chunk for all of HMPRE.
> If this fails we half the chunk size on each iteration and try again.
> 
> On my hardware we start with a chunk_size of 4MiB and just allocate
> 8 (hmmaxd) * 4 = 32 MiB which is worse than 1 * 200MiB.

And that is because the hardware only has a limited set of descriptors.

Is there any real problem you are fixing with this?  Do you actually
see a performance difference on a relevant workload?