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SubjectRe: [PATCH 0/9] RFC: NVME VFIO mediated device [BENCHMARKS]
On Mon, Mar 25, 2019 at 08:52:32PM +0200, Maxim Levitsky wrote:
> Hi
>
> This is first round of benchmarks.
>
> The system is Intel(R) Xeon(R) Gold 6128 CPU @ 3.40GHz
>
> The system has 2 numa nodes, but only cpus and memory from node 0 were used to
> avoid noise from numa.
>
> The SSD is Intel® Optane™ SSD 900P Series, 280 GB version
>
>
> https://ark.intel.com/content/www/us/en/ark/products/123628/intel-optane-ssd-900p-series-280gb-1-2-height-pcie-x4-20nm-3d-xpoint.html
>
>
> ** Latency benchmark with no interrupts at all **
>
> spdk was complited with fio plugin in the host and in the guest.
> spdk was first run in the host
> then vm was started with one of spdk,pci passthrough, mdev and inside the
> vm spdk was run with fio plugin.
>
> spdk was taken from my branch on gitlab, and fio was complied from source for
> 3.4 branch as needed by the spdk fio plugin.
>
> The following spdk command line was used:
>
> $WORK/fio/fio \
> --name=job --runtime=40 --ramp_time=0 --time_based \
> --filename="trtype=PCIe traddr=$DEVICE_FOR_FIO ns=1" --ioengine=spdk \
> --direct=1 --rw=randread --bs=4K --cpus_allowed=0 \
> --iodepth=1 --thread
>
> The average values for slat (submission latency), clat (completion latency) and
> its sum (slat+clat) were noted.
>
> The results:
>
> spdk fio host:
> 573 Mib/s - slat 112.00ns, clat 6.400us, lat 6.52ms
> 573 Mib/s - slat 111.50ns, clat 6.406us, lat 6.52ms
>
>
> pci passthough host/
> spdk fio guest
> 571 Mib/s - slat 124.56ns, clat 6.422us lat 6.55ms
> 571 Mib/s - slat 122.86ns, clat 6.410us lat 6.53ms
> 570 Mib/s - slat 124.95ns, clat 6.425us lat 6.55ms
>
> spdk host/
> spdk fio guest:
> 535 Mib/s - slat 125.00ns, clat 6.895us lat 7.02ms
> 534 Mib/s - slat 125.36ns, clat 6.896us lat 7.02ms
> 534 Mib/s - slat 125.82ns, clat 6.892us lat 7.02ms
>
> mdev host/
> spdk fio guest:
> 534 Mib/s - slat 128.04ns, clat 6.902us lat 7.03ms
> 535 Mib/s - slat 126.97ns, clat 6.900us lat 7.03ms
> 535 Mib/s - slat 127.00ns, clat 6.898us lat 7.03ms
>
>
> As you see, native latency is 6.52ms, pci passthrough barely adds any latency,
> while both mdev/spdk added about (7.03/2 - 6.52) - 0.51ms/0.50ms of latency.

Milliseconds is surprising. The SSD's spec says 10us read/write
latency. Did you mean microseconds?

>
> In addtion to that I added few 'rdtsc' into my mdev driver to strategically
> capture the cycle count it takes it to do 3 things:
>
> 1. translate a just received command (till it is copied to the hardware
> submission queue)
>
> 2. receive a completion (divided by the number of completion received in one
> round of polling)
>
> 3. deliver an interupt to the guest (call to eventfd_signal)
>
> This is not the whole latency as there is also a latency between the point the
> submission entry is written and till it is visible on the polling cpu, plus
> latency till polling cpu gets to the code which reads the submission entry,
> and of course latency of interrupt delivery, but the above measurements mostly
> capture the latency I can control.
>
> The results are:
>
> commands translated : avg cycles: 459.844 avg time(usec): 0.135
> commands completed : avg cycles: 354.61 avg time(usec): 0.104
> interrupts sent : avg cycles: 590.227 avg time(usec): 0.174
>
> avg time total: 0.413 usec
>
> All measurmenets done in the host kernel. the time calculated using tsc_khz
> kernel variable.
>
> The biggest take from this is that both spdk and my driver are very fast and
> overhead is just a thousand of cpu cycles give it or take.

Nice!

Stefan
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