[Bug 1123272] Re: high cpu usage on download (not optimised SHA256, SHA512, SHA1)

[Julian Andres Klode]

Of course you don't have a CPU bottleneck with curl, it does not run 3
or 4 hashes over everything it downloads. APT needs to hash its
downloads to ensure they are secure, and it uses all available hashes to
do so, so if one hash's security is compromised, we can still hopefully
still rely on the others.

And the topic of this bug report is indeed hash CPU usage, and I
compared our CPU usage on a 1 GB test file in a tmpfs to the CPU usage
of OpenSSL and Nettle. So this test operates on a data size 5 times
higher than the usual packages and under optimal conditions to evaluate
how fast we can hash.

The test shows that on a 1 Gbit/s connection you'll likely be throttled
slightly at a comparable CPU (assuming the connection reaches about 800
Mbit/s, that is, 80% efficiency). If you have a data rate of about 500
Mbit/s, you will likely be fine (not counting parallel downloads).

If there are other problems reducing the download speeds, these are
separate bugs. This one covers the overhead of hashing, nothing else.

BTW: The original bug report talks about 20% CPU usage on a 5 year old
CPU, that seems entirely reasonable and not really an issue. If your
bandwidth is high, you'll have higher CPU usage for a shorter time (like
100% for 5 seconds or so).

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https://bugs.launchpad.net/bugs/1123272

Title:
  high cpu usage on download (not optimised SHA256, SHA512, SHA1)

Status in apt package in Ubuntu:
  Confirmed

Bug description:
  http process takes about 20% on intel i5-3317. Here is output of ps aux:
  root      4309 21.7  0.0  37108  2344 pts/2    S+   18:17   0:01 /usr/lib/apt/methods/http

  it can be easy reproduces with "apt-get download firefox-dbg" or any
  other upgrade realted command.

  The reason of this  overhead are this functions:
  22,34%  http  libapt-pkg.so.4.12.0  [.] SHA256_Transform(_SHA256_CTX*, unsigned int const*)
  14,81%  http  libapt-pkg.so.4.12.0  [.] SHA512_Transform(_SHA512_CTX*, unsigned long const*)
  8,11%  http  libapt-pkg.so.4.12.0  [.] SHA1Transform(unsigned int*, unsigned char const*) 
  5,62%  http  libapt-pkg.so.4.12.0  [.] MD5Transform(unsigned int*, unsigned int const*)

  libapt uses own implementation of this hasches without any optimisation.  Are there any reason why libapt do not use
  openssl? Beside current version of openssl has AVX optimised SHA* implementations.

  ProblemType: Bug
  DistroRelease: Ubuntu 13.04
  Package: apt 0.9.7.7ubuntu1
  ProcVersionSignature: Ubuntu 3.8.0-4.9-generic 3.8.0-rc6
  Uname: Linux 3.8.0-4-generic x86_64
  ApportVersion: 2.8-0ubuntu4
  Architecture: amd64
  Date: Tue Feb 12 18:12:33 2013
  InstallationDate: Installed on 2012-09-13 (152 days ago)
  InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Alpha amd64 (20120913.1)
  MarkForUpload: True
  ProcEnviron:
   TERM=xterm
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=de_DE.UTF-8
   SHELL=/bin/bash
  SourcePackage: apt
  UpgradeStatus: Upgraded to raring on 2013-02-06 (5 days ago)

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