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Artifact downloading from Hydra buffers the whole artifact into RAM before sending it #1357

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delroth opened this issue Feb 5, 2024 · 1 comment
Open

Artifact downloading from Hydra buffers the whole artifact into RAM before sending it #1357

delroth opened this issue Feb 5, 2024 · 1 comment
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@delroth
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delroth commented Feb 5, 2024

hydra/src/lib/Hydra/Controller/Build.pm

Line 239 in c1bd50a

$c->stash->{'plain'} = { data => grab(cmd => ["nix", "--experimental-features", "nix-command",

The way Hydra artifacts downloads works is that it runs "nix store cat", reads the whole contents into RAM, and then sends it as HTTP response. This has multiple issues for larger files (e.g. disk images):

  • Very long time to first byte, which could cause some clients to time out.
  • Very high RAM usage by the Hydra web workers.
  • Significant decrease in throughput, since the transfer time happens ~ 2x.

We noticed it a few weeks ago on #infra:nixos.org but then this got rediscovered again today on #hydra:nixos.org so I thought it might be worth filing a bug to actually track this.
@delroth delroth added the bug label Feb 5, 2024
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delroth commented Apr 12, 2024

This sometimes ends up hogging all workers on h.n.o, causing the website to become unavailable and channel scripts to fail.

Ma27 pushed a commit to Ma27/hydra that referenced this issue Aug 17, 2024
@71rd @yu-re-ka
Stream files from store instead of buffering them
459aa0a 
When an artifact is requested from hydra the output is first copied
from the nix store into memory and then sent as a response, delaying
the download and taking up significant amounts of memory.

As reported in NixOS#1357

Instead of calling a command and blocking while reading in the entire
output, this adds read_into_socket(). the function takes a
command, starting a subprocess with that command, returning a file
descriptor attached to stdout.
This file descriptor is then by responsebuilder of Catalyst to steam
the output directly
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