linux-kernel - Re: [PATCH 10/10] dmapool: improve scalability of dma_pool

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Message-ID: <803feeab-b27b-983d-45da-02a0daf0179a@cybernetics.com>
Date:   Tue, 31 May 2022 18:10:38 -0400
From:   Tony Battersby <tonyb@...ernetics.com>
To:     Keith Busch <kbusch@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, kernel-team@...com,
        Matthew Wilcox <willy@...radead.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH 10/10] dmapool: improve scalability of dma_pool_free

On 5/31/22 17:54, Keith Busch wrote:
> On Tue, May 31, 2022 at 02:23:44PM -0400, Tony Battersby wrote:
>> dma_pool_free() scales poorly when the pool contains many pages because
>> pool_find_page() does a linear scan of all allocated pages.  Improve its
>> scalability by replacing the linear scan with a red-black tree lookup.
>> In big O notation, this improves the algorithm from O(n^2) to O(n * log n).
> The improvement should say O(n) to O(log n), right?

That would be the improvement for a single call to dma_pool_alloc or
dma_pool_free, but I was going with the improvement for "n" calls
instead, which is consistent with the improvement for the example in the
cover letter for mpt3sas.  I would have to look up the convention to be
sure of the proper notation in a situation like this, but I usually
think "inserting N items takes N^2 time"; to me it makes less sense to
say "inserting 1 item takes N time", because the N seems to come out of
nowhere.

Tony