linux-kernel - Re: [PATCH v2 0/7] Improve swiotlb performance by using physical addresses

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Date:	Fri, 12 Oct 2012 09:05:10 -0700
From:	Alexander Duyck <alexander.h.duyck@...el.com>
To:	konrad.wilk@...cle.com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, rob@...dley.net, akpm@...ux-foundation.org,
	joerg.roedel@....com, bhelgaas@...gle.com, shuahkhan@...il.com,
	fujita.tomonori@....ntt.co.jp
CC:	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2 0/7] Improve swiotlb performance by using physical
 addresses

On 10/11/2012 01:34 PM, Alexander Duyck wrote:
> While working on 10Gb/s routing performance I found a significant amount of
> time was being spent in the swiotlb DMA handler. Further digging found that a
> significant amount of this was due to virtual to physical address translation
> and calling the function that did it. It accounted for nearly 60% of the
> total swiotlb overhead.
>
> This patch set works to resolve that by replacing the io_tlb_start and
> io_tlb_end virtual addresses with a physical addresses. In addition it changes
> the io_tlb_overflow_buffer from a virtual to a physical address. I followed
> through with the cleanup to the point that the only functions that really
> require the virtual address for the DMA buffer are the init, free, and
> bounce functions.
>
> In the case of devices that are using the bounce buffers these patches should
> result in only a slight performance gain if any. This is due to the locking
> overhead required to map and unmap the buffers.
>
> In the case of devices that are not making use of bounce buffers these patches
> can significantly reduce their overhead. In the case of an ixgbe routing test
> for example, these changes result in 7 fewer calls to __phys_addr and
> allow is_swiotlb_buffer to become inlined due to a reduction in the number of
> instructions. When running a routing throughput test using small packets I
> saw roughly a 6% increase in packets rates after applying these patches. This
> appears to match up with the CPU overhead reduction I was tracking via perf.
>
> Before:
> Results 10.0Mpps
>
> After:
> Results 10.6Mpps
>
> Finally, I updated the parameter names for several of the core function calls
> as there was some ambiguity in naming. Specifically virtual address pointers
> were named dma_addr. When I changed these pointers to physical I instead used
> the name tlb_addr as this value represented a physical address in the
> io_tlb_start region and is less likely to be confused with a bus address.
>
> v2:
> I reviewed the changes and realized that the first patch that was dropping
> io_tlb_end and calculating the value didn't actually gain me much once I had
> gone through and translated the rest of the addresses to physical addresses.
> As such I have updated the patch so that it instead is converting io_tlb_end
> from a virtual address to a physical address.  This actually helps to reduce
> the overhead for is_swiotlb_buffer and swiotlb_dma_supported by several
> instructions.

The first three patches were replacing "static char *" with
"phys_addr_t" when I should have been using "static phys_addr_t".  As
such I will submit an updated v3 version on Monday.

Thanks,

Alex
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