Optimise alert ingest path

[gouthamve]

From @brancz :
While benchmarking & profiling Alertmanager it quickly became obvious, that the ingest path blocks a lot

Can you share some numbers about lock wait times?

[brancz]

Need to run the benchmarks again, for exact numbers. I remember that the channel alerts are sent to in the dispatcher had the longest wait times. Increasing the channel buffer improved this a bit, but obviously only pushes out the issue.

A question we need to ask ourselves as well is, what kind of load are we expecting Alertmanager to handle? (nonetheless the limit should be resource bound not a technical limitation)

[brian-brazil]

I think a million active alerts is a reasonable starting point. See also prometheus/prometheus#2585

[starsdeep]

Hey! I am a graduate student who want to apply GSoC this summer. I previously have some Go experience and I am seeking a Go performance optimization related project.

The problem here is the dispatcher can not send out alert messages to all kinds of client efficiently, or data can not write into dispatcher efficiently?

[brian-brazil]

It's everything before the dispatcher that needs optimisation.

[stuartnelson3]

I think a million active alerts is a reasonable starting point.

What sort of request rate are you expecting for these 1 million alerts? In what little testing I've done, AM had no issue maintaining several thousands of active alerts (< 50,000), but had issues processing incoming alert batches when the rate hit ~30 requests/sec (the issue could occur at a lower rate, this was just the rate I noticed at which AM locked up).

[brian-brazil]

With a 10s eval interval it'd be 100k alerts/s which with a batch size of 64 would be ~1.5k requests/s. prometheus/prometheus#2585 can bring that down to ~250/s.

[starsdeep]

Hey, I tried to use Go pprof to profiling AlertManager, but this seems can only help us to locate some inefficient function implementation. The whole workflow of AlertManager includes: deduplicating, grouping, inhibition, and routing, if we want to locate which stage is the bottleneck in high concurrency case, seems we need manually write code to track and time functions?

@brancz Do you have some benchmark code to share?

[brancz]

@starsdeep I had previously built and used this, I remember having taken mutex profiles and seen a lot of lock contention starting at the dispatcher.

[starsdeep]

@brancz I tried to use the ambench code to launch a benchmark, there are some goroutine block events though, there is no mutex contention events:

PS: I launch AlertManager instances using "goreman start" with "DEBUG=true" environment variable, and run load test with ./ambench -alertmanagers=http://localhost:9093,http://localhost:9094,http://localhost:9095

[brancz]

what does the block profile look like? 🙂

[starsdeep]

@brancz

[brancz]

I would look closer at that Dispatcher 90s that's in the ingest path of alerts being added through the API.