Uncovering performance regressions in the TCP SACKs vulnerability fixes | Hacker News Reader

Uncovering performance regressions in the TCP SACKs vulnerability fixes

databricks.com

32 pointsrxin3 comments

This is a fun write up to read, because I experienced exactly the same problem in a similar system dealing with lots of writes to S3. Sporadic 15 minute timeouts for no immediate reason, especially as the files were only a few megabytes in size.

It led to me going on a similar journey of diving deep into the stack right into doing diffs on the kernel tree to work out what had changed between kernel versions. Eventually I came to the same conclusion, and only recently the problem has been patched in CENTOS6/RHEL6 https://access.redhat.com/errata/RHSA-2019:2736

Interestingly after identifying this problem, I also noticed similar behaviour on AWS Lambda shortly after June 20th, with the TCPWQueueTooBig metric spiking and causing Lambda timeouts. Took a few rounds through AWS support (and our account managers) to get them to look at it, but they eventually fixed it.

I think the common thread between this post and my experience is we are both using a Java/JVM based stack. When trying to reproduce the bug for Amazon I could only reproduce it with a simple Java example, whereas my attempt with Golang seemed to run fine - so not really sure why that was.

Maybe I'll write a similar blog about my findings, at least I learnt a lot from it!

I'd be very interested in if you find something specific. Do you have any details regarding the AWS contacts because i'd like to see if i can generalize it for the RHEL6 case and maybe feed it back into engineering.

wmealing @ redhat dawt com.

Chris from Databricks here.

Glad you enjoyed the write up and glad to hear we aren’t alone.

We also had difficulty creating a repro outside of Spark (JVM). I tried with Python sockets without any luck. That said, hitting the issue requires the right mix of dropped packets, socket buffer sizes and MSS. I don’t think there is anything special about the JVM influencing those variables. Now that I know more, maybe I can craft a minimal repro in another language.

A datapoint I didn’t mention in the post is that we had a significantly higher repro rate when talking to S3 through a VPC endpoint. The only difference I could see was that the VPC endpoint connections had an MSS of 1412, while the MSS was slightly higher (1436 IIRC) on non-VPC connections. Yet to draw conclusions from that.

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