Posted on 13/06/2017 by Helen Koike
When I last wrote about NVMe, the feature to improve NVMe performance over emulated environments was just a living discussion and a work in progress patch. However, it has now been officially released in the NVMe Specification Revision 1.3 under the name "Doorbell Buffer Config command", along with an implementation that is already in the mainline Linux kernel! \o/
You can already feel the difference in performance if you compile Kernel 4.12-rc1 (or later) and run it over a virtual machine hosted on Google Compute Engine. Google actually updated their hypervisor as soon as the feature was ratified by the NVMe working group, even before it was publicly released.
There were very few changes from the original proposal, I.e. opcodes, return values and now fancy names; the buffers (as described in my last post) are now called Shadow Doorbell and EventIdx buffers.
In short, the first one mimics the Doorbell registers in memory, allowing the emulated controller to fetch the Doorbell value when convenient instead of waiting for the Doorbell register to be written. For its part, the EventIdx provides a hint given by the emulated controller to tell the host if the Doorbell register needs to be updated (in case the emulated controller is not fetching the Doorbell value from the Shadow Doorbell buffer). You can check section 7.13 of the specification for an example of usage.
The following test results were obtained in a machine of type n1-standard-4 (4 vCPUs, 15 GB memory) at Google Cloud Engine platform with Kernel 4.12.0-rc5 using the following command:
$ sudo fio --time_based --name=benchmark --runtime=30 \ --filename=/dev/nvme0n1 --nrfiles=1 --ioengine=libaio --iodepth=32 \ --direct=1 --invalidate=1 --verify=0 --verify_fatal=0 --numjobs=1 \ --rw=randread --blocksize=4k --randrepeat=0
Results (in Input/Ouput Operations per Second):
Without Shadow Doorbell and EventIdx buffers: 43.9K IOPS
With Shadow Doorbell and EventIdx buffers: 184K IOPS
Gain ~= 4 times
Screenshot - Without Shadow Doorbell and EventIdx buffers
Screenshot - With Shadow Doorbell and EventIdx buffers
Enjoy your enhanced numbers of IOPS! :D
Did you know you could register your own PC, or a spare laptop collecting dust in a drawer, to get instant CI going on GitLab? Not only…
For the last month or so, I've been playing with a new project during my work at Collabora, and as I've already briefly talked about at…
For projects of any value and significance, having a comprehensive automated test suite is nowadays considered a standard software engineering…
After I started working for Collabora in April, I've finally been able to put some time on maintenance and development of Geoclue again.…
Like all software, Open Source software isn't without it's bugs and issues. However, thanks to the nature of Open Source, resolving or mitigating…
Last month, the first "MicroDebConf" took place at the Gama campus of the University of Brasilia. Here's a look at how this one day event…