linux-kernel - Re: [PATCH] iommu/dma: Zero pages manually in a length of scatterlist

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Date:   Sun, 4 Nov 2018 07:50:01 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     joro@...tes.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/dma: Zero pages manually in a length of scatterlist

On Thu, Nov 01, 2018 at 02:35:00PM -0700, Nicolin Chen wrote:
> The __GFP_ZERO will be passed down to the generic page allocation
> routine which zeros everything page by page. This is safe to be a
> generic way but not efficient for iommu allocation that organizes
> contiguous pages using scatterlist.
> 
> So this changes drops __GFP_ZERO from the flag, and adds a manual
> memset after page/sg allocations, using the length of scatterlist.
> 
> My test result of a 2.5MB size allocation shows iommu_dma_alloc()
> takes 46% less time, reduced from averagely 925 usec to 500 usec.

And in what case does dma_alloc_* performance even matter?