bug#41702: `guix environment` performance issues

[Lars-Dominik Braun]

Hey,

> That’s over SSH, right?
correct, the worst possible case: Inside two VM’s on a Laptop, SSH transport
between them and /gnu+/var/guix on an NFS share (nfsd is in the same VM as
guix-daemon).

> Probably what’s killing us is the round-trip time for all these small
> RPCs.  We would need pipelining but the RPC protocol is not designed to
> make that easy.
That would have been my best guess too, but it does not seem to be the biggest
problem right now. Looking at the numbers again (both patches applied) with the
attached manifest[1], I see that:

---snip---
Local UNIX socket with and without --no-grafts:
    N           Min           Max        Median           Avg        Stddev
x  10          6.07          6.35         6.145          6.16    0.08232726
+  10         17.47         17.89        17.545        17.602    0.14351152
Difference at 99.0% confidence
        11.442 +/- 0.150576
        185.747% +/- 4.07133%

Local UNIX socket vs. guix://localhost transport:
    N           Min           Max        Median           Avg        Stddev
x  10         17.47         17.89        17.545        17.602    0.14351152
+  10         17.43          18.1         17.61        17.642    0.20131788
No difference proven at 99.0% confidence

Local UNIX socket vs ssh://localhost transport:
    N           Min           Max        Median           Avg        Stddev
x  10         17.47         17.89        17.545        17.602    0.14351152
+  10         33.46         35.27        34.315        34.359    0.53873205
Difference at 99.0% confidence
        16.757 +/- 0.5074
        95.1994% +/- 3.13957%
---snap---

So I would conclude:

1) Grafting still takes a lot of time and needs more work
2) Linux optimizes localhost networking pretty well
3) Our SSH transport is terribly slow

Moving to non-localhost communication between two VM’s:

---snip---
guix://localhost vs. guix://remote-host transport:
    N           Min           Max        Median           Avg        Stddev
x  10         17.43          18.1         17.61        17.642    0.20131788
+  10         20.88         22.58        21.095        21.222    0.49689704
Difference at 99.0% confidence
        3.58 +/- 0.487934
        20.2925% +/- 2.85159%

guix://remote-host vs. ssh://remote-host:
    N           Min           Max        Median           Avg        Stddev
x  10         20.88         22.58        21.095        21.222    0.49689704
+  10          30.1         32.56        31.005        31.093    0.70740606
Difference at 99.0% confidence
        9.871 +/- 0.786769
        46.5131% +/- 4.35326%
---snap---

Conclusion here is the same: Not alot of impact of networking/NFS and SSH
transport is still terribly slow. (Confusingly faster than localhost though.)

> Perhaps you could “strace -Tt” the thing to check whether this
> hypothesis is correct by looking at the time we spend waiting for
> replies?
I’m not sure this will yield meaningful data for SSH, so I analyzet it for
guix://localhost vs. guix://remote-host. Takeaway is, yes, of course there is a
statistically significant difference and it’s about 40%±50%, which means this
method is pretty useless, because we can’t bin RPC’s by type.

So, I guess it would make sense for me to look at the SSH transport itself
again and see if there are any other low-hanging fruit. Not sure how much I can
help with profiling guile/guix itself. A different/better RPC protocol is
probably GSoC/v2.0-worthy?

Sorry for all the lengthy emails,
Lars

[1] You’ll need this channel: https://github.com/leibniz-psychology/guix-zpid

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