On Fri, Feb 10, 2023 at 10:18:08AM -0800, Bart Van Assche wrote:
On 2/10/23 10:00, Kanchan Joshi wrote:
3. DMA cost: is high in presence of IOMMU. Keith posted the work[1],
with block IO path, last year. I imagine plumbing to get a bit simpler
with passthrough-only support. But what are the other things that must
be sorted out to have progress on moving DMA cost out of the fast path?
Are performance numbers available?
Around 55% decline when I checked last (6.1-rcX kernel).
512b randread IOPS with optane, on AMD ryzen 9 box -
when iommu is set to lazy (default config)= 3.1M
when iommmu is disabled or in passthrough mode = 4.9M
Isn't IOMMU cost something that has already been solved? From https://www.usenix.org/system/files/conference/atc15/atc15-paper-peleg.pdf:
"Evaluation of our designs under Linux shows that (1)
they achieve 88.5%–100% of the performance obtained
without an IOMMU".
Since above numbers are more recent than the paper, this is yet to be
solved.
