Re: Intel IOMMU (and IOMMU for Virtualization) performances

[mark gross <mgross@linux.intel.com>]

On Fri, Jun 06, 2008 at 01:44:30PM +0900, FUJITA Tomonori wrote:
> On Thu, 5 Jun 2008 15:02:16 -0700
> mark gross <mgross@linux.intel.com> wrote:
> 
> > 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.
> 
> Yeah, I know. I'll test 'intel_iommu=strict' option next time.
> 
> The patch also has 'intel_iommu=strict' option. Wiht it enabled, it
> flushes TLB cache every time dma_unmap_* is called as the original
> code does.
> 
> 
> > > 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.  
> 
> Why did you choose the RB tree instead of a traditional bitmap scheme
> to manage free space?
>

I inherited this code.  And I'm passing it on to David Woodhouse soon.
I don't know why RB was used over BM.  I guess for scalability to many
10Gig IO devices, but thats just a guess.
 
> 
> > > 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.
> 
> Thanks a lot!