On Wed, Jun 04, 2008 at 11:47:01PM +0900, FUJITA Tomonori wrote: > I resumed the work to make the IOMMU respect drivers' DMA alignment > (since I got a desktop box having VT-d). In short, some IOMMUs > allocate memory areas spanning driver's segment boundary limit (DMA > alignment). It forces drivers to have a workaround to split up scatter > entries into smaller chunks again. To remove such work around in > drivers, I modified several IOMMUs, X86_64 (Calgary and Gart), Alpha, > POWER, PARISC, IA64, SPARC64, and swiotlb. > > Now I try to fix Intel IOMMU code, the free space management > algorithm. > > The major difference between Intel IOMMU code and the others is Intel > IOMMU code uses Red Black tree to manage free space while the others > use bitmap (swiotlb is the only exception). > > The Red Black tree method consumes less memory than the bitmap method, > but it incurs more overheads (the RB tree method needs to walk through > the tree, allocates a new item, and insert it every time it maps an > I/O address). Intel IOMMU (and IOMMUs for virtualization) needs > multiple IOMMU address spaces. That's why the Red Black tree method is > chosen, I guess. > > Half a year ago, I tried to convert POWER IOMMU code to use the Red > Black method and saw performance drop: > > http://linux.derkeiler.com/Mailing-Lists/Kernel/2007-11/msg00650.html > > So I tried to convert Intel IOMMU code to use the bitmap method to see > how much I can get. > > I didn't see noticable performance differences with 1GbE. So I tried > the modified driver of a SCSI HBA that just does memory accesses to > emulate the performances of SSD disk drives, 10GbE, Infiniband, etc. > > I got the following results with one thread issuing 1KB I/Os: > > IOPS (I/O per second) > IOMMU disabled 145253.1 (1.000) > RB tree (mainline) 118313.0 (0.814) > Bitmap 128954.1 (0.887) > FWIW: You'll see bigger deltas if you boot with intel_iommu=strict, but those will be because of waiting on IOMMU hardware to flush caches and may further hide effects of gong with a bitmap as opposed to a RB tree. > > I've attached the patch to modify Intel IOMMU code to use the bitmap > method but I have no intention of arguing that Intel IOMMU code > consumes more memory for better performance. :) I want to do more > performance tests with 10GbE (probably, I have to wait for a server > box having VT-d, which is not available on the market now). > > As I said, what I want to do now is to make Intel IOMMU code respect > drivers' DMA alignment. Well, it's easier to do that if Intel IOMMU > uses the bitmap method since I can simply convert the IOMMU code to > use lib/iommu-helper but I can modify the RB tree method too. > I'm going to be out of contact for a few weeks but this work sounds interesting. > I'm just interested in other people's opinions on IOMMU > implementations, performances, possible future changes for performance > improvement, etc. > > For further information: > > LSF'08 "Storage Track" summary by Grant Grundler: > http://iou.parisc-linux.org/lsf2008/SUMMARY-Storage.txt > > My LSF'08 slides: > http://iou.parisc-linux.org/lsf2008/IO-DMA_Representations-fujita_tomonori.pdf > > > Tis patch is against the latst git tree (note that it just converts > Intel IOMMU code to use the bitmap. It doesn't make it respect > drivers' DMA alignment yet). > I'll look closely at your patch later. --mgross -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
