Allows for remote monitoring of a running process over HTTP.

This module can be used to run an HTTP server that replies to HTTP requests with either an HTML page or a JSON object. The former can be used by a human to get an overview of a program's GC stats and the latter can be used be automated tools.

Typical usage is to start the monitor server on program startup

 main = do
     forkServer "localhost" 8000
     ...

and then periodically check the stats using a web browser or a command line tool like curl

 $ curl -H "Accept: application/json" http://localhost:8000/

To use this module you must first enable GC stats collection for your program. To enable GC stats collection, either run your program with

 +RTS -T

or compile it with

 -with-rtsopts=-T

The runtime overhead of -T is very small so it's safe to always leave it enabled.

The HTTP server replies to GET requests to the host and port passed to forkServer. To get a JSON formatted response, the client must set the Accept header to "application/json". The server returns a JSON object with the following members:

bytes_allocated

Total number of bytes allocated

num_gcs

Number of garbage collections performed

max_bytes_used

Maximum number of live bytes seen so far

num_bytes_usage_samples

Number of byte usage samples taken

cumulative_bytes_used

Sum of all byte usage samples, can be used with numByteUsageSamples to calculate averages with arbitrary weighting (if you are sampling this record multiple times).

bytes_copied

Number of bytes copied during GC

current_bytes_used

Current number of live bytes

current_bytes_slop

Current number of bytes lost to slop

max_bytes_slop

Maximum number of bytes lost to slop at any one time so far

peak_megabytes_allocated

Maximum number of megabytes allocated

mutator_cpu_seconds

CPU time spent running mutator threads. This does not include any profiling overhead or initialization.

mutator_wall_seconds

Wall clock time spent running mutator threads. This does not include initialization.

gc_cpu_seconds

CPU time spent running GC

gc_wall_seconds

Wall clock time spent running GC

cpu_seconds

Total CPU time elapsed since program start

wall_seconds

Total wall clock time elapsed since start

par_avg_bytes_copied

Number of bytes copied during GC, minus space held by mutable lists held by the capabilities. Can be used with parMaxBytesCopied to determine how well parallel GC utilized all cores.

par_max_bytes_copied

Sum of number of bytes copied each GC by the most active GC thread each GC. The ratio of parAvgBytesCopied divided by parMaxBytesCopied approaches 1 for a maximally sequential run and approaches the number of threads (set by the RTS flag -N) for a maximally parallel run.