On 12/10/20 8:56 AM, xuxiaoyang (C) wrote:
On 2020/12/9 22:42, Eric Farman wrote:
On 12/9/20 6:54 AM, Cornelia Huck wrote:
On Tue, 8 Dec 2020 21:55:53 +0800
"xuxiaoyang (C)" <xuxiaoyang2@xxxxxxxxxx> wrote:
On 2020/11/21 15:58, xuxiaoyang (C) wrote:
vfio_pin_pages() accepts an array of unrelated iova pfns and processes
each to return the physical pfn. When dealing with large arrays of
contiguous iovas, vfio_iommu_type1_pin_pages is very inefficient because
it is processed page by page.In this case, we can divide the iova pfn
array into multiple continuous ranges and optimize them. For example,
when the iova pfn array is {1,5,6,7,9}, it will be divided into three
groups {1}, {5,6,7}, {9} for processing. When processing {5,6,7}, the
number of calls to pin_user_pages_remote is reduced from 3 times to once.
For single page or large array of discontinuous iovas, we still use
vfio_pin_page_external to deal with it to reduce the performance loss
caused by refactoring.
Signed-off-by: Xiaoyang Xu <xuxiaoyang2@xxxxxxxxxx>
(...)
hi Cornelia Huck, Eric Farman, Zhenyu Wang, Zhi Wang
vfio_pin_pages() accepts an array of unrelated iova pfns and processes
each to return the physical pfn. When dealing with large arrays of
contiguous iovas, vfio_iommu_type1_pin_pages is very inefficient because
it is processed page by page. In this case, we can divide the iova pfn
array into multiple continuous ranges and optimize them. I have a set
of performance test data for reference.
The patch was not applied
1 page 512 pages
no huge pages： 1638ns 223651ns
THP： 1668ns 222330ns
HugeTLB： 1526ns 208151ns
The patch was applied
1 page 512 pages
no huge pages 1735ns 167286ns
THP： 1934ns 126900ns
HugeTLB： 1713ns 102188ns
As Alex Williamson said, this patch lacks proof that it works in the
real world. I think you will have some valuable opinions.
Looking at this from the vfio-ccw angle, I'm not sure how much this
would buy us, as we deal with IDAWs, which are designed so that they
can be non-contiguous. I guess this depends a lot on what the guest
does.
This would be my concern too, but I don't have data off the top of my head to say one way or another...
Eric, any opinion? Do you maybe also happen to have a test setup that
mimics workloads actually seen in the real world?
...I do have some test setups, which I will try to get some data from in a couple days. At the moment I've broken most of those setups trying to implement some other stuff, and can't revert back at the moment. Will get back to this.
Eric
.
Thank you for your reply. Looking forward to your test data.
Xu,
The scenario I ran was a host kernel 5.10.0-rc7 with qemu 5.2.0, with a
Fedora 32 guest with 4 VCPU and 4GB memory. I tried this a handful of
times across a couple different hosts, so the likelihood that these
numbers are outliers are pretty low. The histograms below come from a
simple bpftrace, recording the number of pages asked to be pinned, and
the length of time (in nanoseconds) it took to pin all those pages. I
separated out the length of time for a request of one page versus a
request of multiple pages, because as you will see the former far
outnumbers the latter.
The first thing I tried was simply to boot the guest via vfio-ccw, to
see how the patch itself behaved:
@1_page_ns BASE +PATCH
256, 512 12531 42.50% 12744 42.26%
512, 1K 5660 19.20% 5611 18.61%
1K, 2K 8416 28.54% 8947 29.67%
2K, 4K 2694 9.14% 2669 8.85%
4K, 8K 164 0.56% 169 0.56%
8K, 16K 14 0.05% 14 0.05%
16K, 32K 2 0.01% 3 0.01%
32K, 64K 0 0.00% 0 0.00%
64K, 128K 0 0.00% 0 0.00%
@n_pages_ns BASE +PATCH
256, 512 0 0.00% 0 0.00%
512, 1K 67 0.97% 48 0.68%
1K, 2K 1598 23.13% 1036 14.71%
2K, 4K 2784 40.30% 3112 44.17%
4K, 8K 1288 18.64% 1579 22.41%
8K, 16K 1011 14.63% 1032 14.65%
16K, 32K 159 2.30% 234 3.32%
32K, 64K 1 0.01% 2 0.03%
64K, 128K 0 0.00% 2 0.03%
@npage BASE +PATCH
1 29484 81.02% 30157 81.06%
2, 4 3298 9.06% 3385 9.10%
4, 8 1011 2.78% 1029 2.77%
8, 16 2600 7.14% 2631 7.07%
The second thing I tried was simply fio, running it for about 10 minutes
with a few minutes each for sequential read, sequential write, random
read, and random write. (I tried this with both the guest booted off
vfio-ccw and virtio-blk, but the difference was negligible.) The results
in this space are similar as well:
@1_page_ns BASE +PATCH
256, 512 5648104 66.79% 6615878 66.75%
512, 1K 1784047 21.10% 2082852 21.01%
1K, 2K 648877 7.67% 771964 7.79%
2K, 4K 339551 4.01% 396381 4.00%
4K, 8K 32513 0.38% 40359 0.41%
8K, 16K 2602 0.03% 2884 0.03%
16K, 32K 758 0.01% 762 0.01%
32K, 64K 434 0.01% 352 0.00%
@n_pages_ns BASE +PATCH
256, 512 0 0.00% 0 0.00%
512, 1K 470803 12.18% 360524 7.95%
1K, 2K 1305166 33.75% 1739183 38.37%
2K, 4K 1338277 34.61% 1471161 32.46%
4K, 8K 733480 18.97% 937341 20.68%
8K, 16K 16954 0.44% 20708 0.46%
16K, 32K 1278 0.03% 2197 0.05%
32K, 64K 707 0.02% 703 0.02%
@npage BASE +PATCH
1 8457107 68.62% 9911624 68.62%
2, 4 2066957 16.77% 2446462 16.94%
4, 8 359989 2.92% 417188 2.89%
8, 16 1440006 11.68% 1668482 11.55%
I tried a smattering of other tests that might be more realistic, but
the results were all pretty similar so there's no point in appending
them here. Across the board, the amount of time spent on a multi-page
request grows with the supplied patch. It doesn't get me very excited.
If you are wondering why this might be, Conny's initial take about IDAWs
being non-contiguous by design is spot on. Let's observe the page counts
given to vfio_iommu_type1_pin_contiguous_pages() in addition to the
counts in vfio_iommu_type1_pin_pages(). The following is an example of
one guest boot PLUS an fio run:
vfio_iommu_type1_pin_pages npage:
1 9890332 68.64%
2, 4 2438213 16.92%
4, 8 416278 2.89%
8, 16 1663201 11.54%
Total 14408024
vfio_iommu_type1_pin_contiguous_pages npage:
1 16384925 86.89%
2, 4 1327548 7.04%
4, 8 727564 3.86%
8, 16 417182 2.21%
Total 18857219
Yup... 87% of the calls to vfio_iommu_type1_pin_contiguous_pages() do so
with a length of just a single page.
Happy to provide more data if desired, but it doesn't look like a
benefit to vfio-ccw's use.
Thanks,
Eric
Regards,
Xu
