Re: [PATCH 0/9] RFC: NVME VFIO mediated device [BENCHMARKS]

[Stefan Hajnoczi]

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 Optaneâ 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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