The Performance Engineering group under direction of John Shakshober (aka Shak), had a very busy spring working with our excellent customer and partner ecosystem, generating high-value content for Summit attendees. A great example of collaboration with customers was a super interesting talk from NASA, along with Red Hat’s Mark Wagner and Shak. Hopefully they post a video of it!

On Red Hat’s website, you can find videos of the keynotes as well as many other excellent presentations. Be sure to check them out here. All in all, a great week…very happy to re-connect with customers, partners and fellow Red Hat associates.

Continue reading “Performance Analysis and Tuning Videos from Red Hat Summit 2013” →

I tweeted recently that %usr is what we wanna do; %sys is what we gotta do…What I meant was to point out that the kernel’s main goals in life are to bring up hardware, and manage access to it on behalf of an application and get out of the way.  
Continue reading ““Baby, we were born to run…” — Every userspace process ever.” →

What is all that %sys time ?  “I never know what she’s _doing_ in there…” Ha!

12:01:35 PM CPU %usr %nice %sys %iowait %irq %soft %idle
12:01:36 PM all 0.08 0.00  3.33 0.00    0.00 5.00  91.59
12:01:36 PM 0   0.00 0.00 40.59 0.00    0.00 59.41  0.00

...

You can instantly find out with ‘perf top’.  In this case (netperf), the kernel is spending time copying skb’s around, mediating between kernel and userspace.  I wrote a bit about this in a previous blog post; the traditional protection ring.

All that copying takes time…precious, precious time.  And CPU cycles; also precious.  And memory bandwidth…etc.

Continue reading “Battle Plan for RDMA over Converged Ethernet (RoCE)” →

I finally got the pony I’ve been asking for.

There’s a very interesting (and impactful) performance optimization coming to RHEL6.4.  For years we’ve had to do this sort of tuning manually, but thanks to the power of open source, this magical feature has been implemented and is headed your way in RHEL6.4 (try it in the beta!)

What is this magical feature…is it a double-rainbow ?  Yes.  All the way.

It’s vhost thread affinity via virsh emulatorpin.

If you’re familiar with the vhost_net network infrastructure added to Linux, it moves the network I/O out of the main qemu userspace thread to a kthread called vhost-$PID (where $PID is the PPID of the main KVM process for the particular guest).  So if your KVM guest is PID 12345, you would also see a [vhost-12345] process.

Anyway…
Continue reading “Big-win I/O performance increase coming to KVM guests in RHEL6.4” →

I am staring at a workload that is zillions upon zillions of very tiny packets, and each one is important.  They've got to get there fast.  As fast as possible.  Nagle:  you are not welcome here.  I am seeing some seemingly random jitter, and it's only on this one system.  <confused>

I need to take apart this stack piece by piece, and test each in isolation.   Let's start at the lowest level possible.  RHEL6 includes a kernel module called pktgen (modprobe pktgen).  This module allows you to create network packets, specify it's attributes and send them at the fastest possible rate with the least overhead.

Using pktgen, I was able to achieve over 3.3M packets per second on a 10GB Solarflare NIC.  These packets do not have any protocol TCP/UDP packet processing overhead.  You can watch the receivers netstat/IP counters, though.

Since these are synthetic packets, you have to give pktgen some basic information in order for the packets to be constructed with enough info to get there they're going.  Things like destination IP/MAC, the number of packets  and their size.  I tested tiny packets, 64bytes (because that's what this workload needs).  I also tested jumbo frames just to be sure I was doing it right.

This brings up a habit of mine worth mentioning; purposely mis-tuning your environment to validate your settings.  A sound practice!
To get to 3.3Mpps, I only had to make one key change.  Use a 10x factor for  clone_skb.  Anything less than 10 lead to fewer packets (a value of zero halved the pps throughput as compared to 10).  Anything more than 10 had no performance benefit, so I'm sticking with 10 for now.


 Continue reading "Generating arbitrary network packets using the pktgen kernel module" →