The m3.medium is terrible

I’ve been doing some testing of various instance types in our staging environment, originally just to see if Amazon’s t2.* line of instances is usable in a real-world scenario. In the end, I found that not only are the t2.mediums viable for what I want them to do, but they’re far better suited than the m3.medium, which I wouldn’t use for anything that you ever expect to reach any load.

Here are the conditions for my test:

  • Rails application (unicorn) fronted by nginx.
  • The number of unicorn processes is controlled by chef, currently set to (CPU count * 2), so a 2 CPU instance has 4 unicorn workers.
  • All instances are running Ubuntu 14.04 LTS (AMI ami-864d84ee for HVM, ami-018c9568 for paravirtual) with kernel 3.13.0-29-generic #53-Ubuntu SMP Wed Jun 4 21:00:20 UTC 2014 x86_64.
  • The test used loader.io to simulate 65 concurrent clients hitting the API (adding products to cart) as fast as possible for 600 seconds (10 minutes).
  • The instances were all behind an Elastic Load Balancer, which routes traffic based on its own algorithm (supposedly the instances with the lowest CPU always gets the next request).

The below charts summarize the findings.

average nginx $request_time
average nginx $request_time

This chart shows each server’s performance as reported by nginx. The values are the average time to service each request and the standard deviation. While I expected the m3.large to outperform the m3.medium, I didn’t expect the difference to be so dramatic. The performance of the t2.medium is the real surprise, however.

#	_sourcehost	_avg	_stddev
1	m3.large	6.30324	3.84421
2	m3.medium	15.88136	9.29829
3	t2.medium	4.80078	2.71403

These charts show the CPU activity for each instance during the test (data as per CopperEgg).

m3.large
m3.large
t2.medium
t2.medium
m3.medium
m3.medium

The m3.medium has a huge amount of CPU steal, which I’m guessing accounts for its horrible performance. Anecdotally, in my own experience m3.medium far more prone to CPU steal than other instance types. Moving from m3.medium to c3.large (essentially the same instance with 2 cpus) eliminates the CPU steal issue. However, since the t2.medium performs as well as the c3.large or m3.large and costs half of the c3.large (or nearly 1/3 of the m3.large) I’m going to try running most of my backend fleet on t2.medium.

I haven’t mentioned the credits system the t2.* instances use for burstable performance, and that’s because my tests didn’t make much of a dent in the credit balance for these instances. The load test was 100x what I expect to see in normal traffic patterns, so the t2.medium with burstable performance seems like an ideal candidate. I might add a couple c3.large to the mix as a backstop in case the credits were depleted, but I don’t think that’s a major risk – especially not in our staging environment.

Edit: I didn’t include the numbers, but the performance seemed to be the consistent whether on hvm or paravirtual instances.

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Using OpenSWAN to connect two VPCs in different AWS regions

Amazon has a pretty decent writeup on how to do this (here), but in trying to establish Postgres replication across regions, I found some weird behavior where I could connect to the port directly (telnet to 5432) but psql (or pg_basebackup) didn’t work. tcpdump showed this:

16:11:28.419642 IP 10.121.11.47.35039 > 10.1.11.254.postgresql: Flags [P.], seq 9:234, ack 2, win 211, options [nop,nop,TS val 11065893 ecr 1811434], length 225
16:11:28.419701 IP 10.121.11.47.35039 > 10.1.11.254.postgresql: Flags [P.], seq 9:234, ack 2, win 211, options [nop,nop,TS val 11065893 ecr 1811434], length 225
16:11:28.421186 IP 10.1.11.254.postgresql > 10.121.11.47.35039: Flags [.], ack 234, win 219, options [nop,nop,TS val 1811520 ecr 11065893,nop,nop,sack 1 {9:234}], length 0
16:11:28.425273 IP 10.1.11.254.postgresql > 10.121.11.47.35039: Flags [P.], seq 2:1377, ack 234, win 219, options [nop,nop,TS val 1811522 ecr 11065893], length 1375
16:11:28.425291 IP 10.1.96.20 > 10.1.11.254: ICMP 10.121.11.47 unreachable - need to frag (mtu 1422), length 556
16:11:28.697397 IP 10.1.11.254.postgresql > 10.121.11.47.35039: Flags [P.], seq 2:1377, ack 234, win 219, options [nop,nop,TS val 1811590 ecr 11065893], length 1375
16:11:28.697438 IP 10.1.96.20 > 10.1.11.254: ICMP 10.121.11.47 unreachable - need to frag (mtu 1422), length 556
16:11:29.241311 IP 10.1.11.254.postgresql > 10.121.11.47.35039: Flags [P.], seq 2:1377, ack 234, win 219, options [nop,nop,TS val 1811726 ecr 11065893], length 1375
16:11:29.241356 IP 10.1.96.20 > 10.1.11.254: ICMP 10.121.11.47 unreachable - need to frag (mtu 1422), length 556
16:11:30.333438 IP 10.1.11.254.postgresql > 10.121.11.47.35039: Flags [P.], seq 2:1377, ack 234, win 219, options [nop,nop,TS val 1811999 ecr 11065893], length 1375
16:11:30.333488 IP 10.1.96.20 > 10.1.11.254: ICMP 10.121.11.47 unreachable - need to frag (mtu 1422), length 556
16:11:32.513418 IP 10.1.11.254.postgresql > 10.121.11.47.35039: Flags [P.], seq 2:1377, ack 234, win 219, options [nop,nop,TS val 1812544 ecr 11065893], length 1375
16:11:32.513467 IP 10.1.96.20 > 10.1.11.254: ICMP 10.121.11.47 unreachable - need to frag (mtu 1422), length 556
16:11:36.881409 IP 10.1.11.254.postgresql > 10.121.11.47.35039: Flags [P.], seq 2:1377, ack 234, win 219, options [nop,nop,TS val 1813636 ecr 11065893], length 1375
16:11:36.881460 IP 10.1.96.20 > 10.1.11.254: ICMP 10.121.11.47 unreachable - need to frag (mtu 1422), length 556

After quite a bit of Google and mucking in network ACLs and security groups, the fix ended up being this:

iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1500

(The above two commands need to be run on both OpenSwan boxes.)