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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/