Re: next: ccache broken for cross-compiling

[Uladzimir Bely <ubely@ilbers.de>]

In the email from Sunday, 10 July 2022 23:51:14 +03 user Jan Kiszka wrote:
> Hi all,
> 
> I've noticed that ccache is not doing its job with current next when
> cross-building, e.g. mc:qemuarm64-bullseye:linux-mainline. It does work
> as expected for native builds (mc:qemuamd64-bullseye:linux-mainline),
> and it works fine with v0.9. Seems like an sbuild regression.
> 
> Jan

I did some checks on my side and it seems that it is not related to cross-
build, but mostlry related to something specific to sbuild and kernel.

Just some statistics (taken from 'ccache -s' output)

1. 'hello' cross-build for 'arm64' on 'next' branch (with sbuild)

First run:
  Hits:             415 / 7479 (5.55 %)

Second run:
  Hits:            7353 / 14958 (49.16 %)

Everything OK, most of requests are hits ((7353 - 415) / 7479 = 92.7%)

2. 'linux-mainline' cross build for 'arm64' on 'next' branch (with sbuild)

First run:
  Hits:             262 / 6208 (4.22 %)

Second run:
  Hits:              644 / 12416 (5.19 %)

Cache usage is relatively low ((644 - 262) / 6208 = 5.3%) compared to 'hello'

3. 'linux-mainline' native build for 'arm64' on 'next' branch (with sbuild)

First run:
  Hits:             262 / 6208 (4.22 %)

Second run:
  Hits:              644 / 12416 (5.19 %)

The result is the same as for cross-build ((644 - 262) / 6208 = 5.3%), so the 
low cache usage problem is not related to cross or native build type.

4. 'linux-mainline' native build for 'arm64' on 'master' branch (without 
sbuild)

First run:
  Hits:             262 / 6208 (4.22 %)

Second run:
  Hits:            6469 / 12416 (52.10 %)

Here, ccache usage is high ((6469 - 262) / 6208 = 99.98%)

Most probably, Henning's assumption regarding inclusion of debug information 
(with different absolute paths in case of sbuild) making binaries in cache 
'incompatible' is true. 

-- 
Uladzimir Bely