Re: [PATCH v2] vfio iommu type1: Improve vfio_iommu_type1_pin_pages performance

[Eric Farman]

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