Hi Christoph, On Sun, Nov 04, 2018 at 07:50:01AM -0800, Christoph Hellwig wrote: > 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?
Honestly, this was amplified by running a local iommu benchmark test. Practically dma_alloc/free() should not be that stressful, but we cannot say the performance doesn't matter at all, right? Though many device drivers pre-allocte memory for DMA usage, it could matter where a driver dynamically allocates and releases. And actually I have a related question for you: I saw that the dma_direct_alloc() cancels the __GFP_ZERO flag and does manual memset() after allocation. Might that be possibly related to a performance concern? Though I don't see any performance keyword for that part of code, especially seems that memset() was there from the beginning. Thanks Nicolin _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
