Re: [RFC PATCH 0/7] Improve swiotlb performance by using physical addresses

[Konrad Rzeszutek Wilk <konrad@kernel.org>]

On Wed, Oct 03, 2012 at 05:38:41PM -0700, 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 the fact that virtual to physical
> address translation and calling the function that did it.  It accounted for
> nearly 60% of the total overhead.
> 
> This patch set works to resolve that by changing the io_tlb_start address and
> io_tlb_overflow_buffer address from virtual addresses to physical addresses.
> By doing this, devices that are not making use of bounce buffers can
> significantly reduce their overhead.  In addition I followed through with the

.. but are still using SWIOTLB for their DMA operations, right?

> 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.
> 
> When running a routing throughput test using small packets I saw roughly a 5%
> 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 10.29Mps
> # Overhead Symbol
> # ........ ...........................................................................................................
> #
>      1.97%  [k] __phys_addr                                                                                   
>             |              
>             |--24.97%-- swiotlb_sync_single
>             |          
>             |--16.55%-- is_swiotlb_buffer
>             |          
>             |--11.25%-- unmap_single
>             |          
>              --2.71%-- swiotlb_dma_mapping_error
>      1.66%  [k] swiotlb_sync_single                                                                                 
>      1.45%  [k] is_swiotlb_buffer                                     
>      0.53%  [k] unmap_single                                                                                     
>      0.52%  [k] swiotlb_map_page                                      
>      0.47%  [k] swiotlb_sync_single_for_device                                                                 
>      0.43%  [k] swiotlb_sync_single_for_cpu                           
>      0.42%  [k] swiotlb_dma_mapping_error                                                               
>      0.34%  [k] swiotlb_unmap_page                
> 
> After:
> Results 10.99Mps
> # Overhead Symbol
> # ........ ...........................................................................................................
> #
>      0.50%  [k] swiotlb_map_page                                                                                        
>      0.50%  [k] swiotlb_sync_single                                                                                     
>      0.36%  [k] swiotlb_sync_single_for_cpu                                                                             
>      0.35%  [k] swiotlb_sync_single_for_device                                                                          
>      0.25%  [k] swiotlb_unmap_page                                                                                      
>      0.17%  [k] swiotlb_dma_mapping_error  
> 
> ---
> 
> Alexander Duyck (7):
>       swiotlb:  Do not export swiotlb_bounce since there are no external consumers
>       swiotlb: Use physical addresses instead of virtual in swiotlb_tbl_sync_single
>       swiotlb: Use physical addresses for swiotlb_tbl_unmap_single
>       swiotlb: Return physical addresses when calling swiotlb_tbl_map_single
>       swiotlb: Make io_tlb_overflow_buffer a physical address
>       swiotlb: Make io_tlb_start a physical address instead of a virtual address
>       swiotlb: Instead of tracking the end of the swiotlb region just calculate it
> 
> 
>  drivers/xen/swiotlb-xen.c |   25 ++---
>  include/linux/swiotlb.h   |   20 ++--
>  lib/swiotlb.c             |  247 +++++++++++++++++++++++----------------------
>  3 files changed, 150 insertions(+), 142 deletions(-)
> 
> -- 
> 
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