shrinking Prometheus binaries

Out of the box, the Prometheus binaries in the latest release (v2.7.1) clock in rather large, at:

-rwxr-xr-x 1 ward ward 60097588 Jan 31 06:18 prometheus
-rwxr-xr-x 1 ward ward 38359349 Jan 31 06:19 promtool

Fillippo Valsorda has a post that describes shrinking golang binaries.

Prometheus uses the promu tool for building its binaries. Adding the necessary ld flags is done by editing .promu.yml file like this:

--- .promu.yml.orig	2019-02-20 08:57:29.413554323 -0500
+++ .promu.yml	2019-02-19 17:44:33.451539440 -0500
@@ -17,6 +17,8 @@
         -X github.com/prometheus/common/version.Branch={{.Branch}}
         -X github.com/prometheus/common/version.BuildUser={{user}}@{{host}}
         -X github.com/prometheus/common/version.BuildDate={{date "20060102-15:04:05"}}
+        -s
+        -w
 tarball:
     files:
         - consoles

After building the prometheus binaries with the latest stable go (1.11.5) using make build, we end up with:

-rwxr-xr-x  1 ward ward 46379136 Feb 19 17:45 prometheus
-rwxr-xr-x  1 ward ward 29391136 Feb 19 17:46 promtool

The binaries are now reduced to about 77% of their original size. Better, but still not great.

We can confirm they are now indeed statically linked and stripped of debug info:

$ file prometheus
prometheus: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, stripped
$ file promtool
promtool: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, stripped

Applying upx takes a while but yields excellent results:

$ upx --brute prometheus 
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2013
UPX 3.91        Markus Oberhumer, Laszlo Molnar & John Reiser   Sep 30th 2013

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
  46379136 ->   9370100   20.20%  linux/ElfAMD   prometheus                    

Packed 1 file.

$ upx --brute promtool
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2013
UPX 3.91        Markus Oberhumer, Laszlo Molnar & John Reiser   Sep 30th 2013

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
  29391136 ->   6271396   21.34%  linux/ElfAMD   promtool                      

Packed 1 file.

And here are the resulting binaries:

-rwxr-xr-x  1 ward ward 9370100 Feb 19 17:45 prometheus
-rwxr-xr-x  1 ward ward 6271396 Feb 19 17:46 promtool

The binaries are now down to roughly 16% of their original size. There is a noticeable startup delay at runtime for the upx decompression, but for a long-running daemon like prometheus that doesn’t matter at all.

For promtool, which is used more interactively, you might find the startup delay annoying. This is the original binary:

$ time ./promtool.orig 
usage: promtool.orig []  [ ...]

Tooling for the Prometheus monitoring system.
...

real	0m0.055s
user	0m0.036s
sys	0m0.013s

And here’s the UPX-compressed one:

$ time ./promtool
usage: promtool []  [ ...]

Tooling for the Prometheus monitoring system.
...

real	0m0.536s
user	0m0.521s
sys	0m0.016s