Re: [PATCH v3 4/6] gitlab-ci: Add ccache in $PATH and display statistics

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

On 5/21/21 2:52 PM, Daniel P. Berrangé wrote:
> On Fri, May 21, 2021 at 02:27:26PM +0200, Philippe Mathieu-Daudé wrote:
>> On 5/21/21 1:53 PM, Daniel P. Berrangé wrote:
>>> On Fri, May 21, 2021 at 01:02:51PM +0200, Thomas Huth wrote:
>>>> On 21/05/2021 12.50, Daniel P. Berrangé wrote:
>>>>> On Fri, May 21, 2021 at 12:48:21PM +0200, Thomas Huth wrote:
>>>>>> On 20/05/2021 13.27, Philippe Mathieu-Daudé wrote:
>>>>>>> +Stefan/Daniel
>>>>>>>
>>>>>>> On 5/20/21 10:02 AM, Thomas Huth wrote:
>>>>>>>> On 19/05/2021 20.45, Philippe Mathieu-Daudé wrote:
>>>>>>>>> If a runner has ccache installed, use it and display statistics
>>>>>>>>> at the end of the build.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
>>>>>>>>> ---
>>>>>>>>>     .gitlab-ci.d/buildtest-template.yml | 5 +++++
>>>>>>>>>     1 file changed, 5 insertions(+)
>>>>>>>>>
>>>>>>>>> diff --git a/.gitlab-ci.d/buildtest-template.yml
>>>>>>>>> b/.gitlab-ci.d/buildtest-template.yml
>>>>>>>>> index f284d7a0eec..a625c697d3b 100644
>>>>>>>>> --- a/.gitlab-ci.d/buildtest-template.yml
>>>>>>>>> +++ b/.gitlab-ci.d/buildtest-template.yml
>>>>>>>>> @@ -6,13 +6,18 @@
>>>>>>>>>           then
>>>>>>>>>             JOBS=$(sysctl -n hw.ncpu)
>>>>>>>>>             MAKE=gmake
>>>>>>>>> +        PATH=/usr/local/libexec/ccache:$PATH
>>>>>>>>>             ;
>>>>>>>>>           else
>>>>>>>>>             JOBS=$(expr $(nproc) + 1)
>>>>>>>>>             MAKE=make
>>>>>>>>> +        PATH=/usr/lib/ccache:/usr/lib64/ccache:$PATH
>>>>>>>>
>>>>>>>> That does not make sense for the shared runners yet. We first need
>>>>>>>> something to enable the caching there - see my series "Use ccache in the
>>>>>>>> gitlab-CI" from April (which is currently stalled unfortunately).
>>>>>>>
>>>>>>> TL;DR: I don't think we should restrict our templates to shared runners.
>>>>>>
>>>>>> I'm certainly not voting for restricting ourselves to only use shared
>>>>>> runners here - but my concern is that this actually *slows* down the shared
>>>>>> runners even more! (sorry, I should have elaborated on that in my previous
>>>>>> mail already)
>>>>>>
>>>>>> When I was experimenting with ccache in the shared runners, I saw that the
>>>>>> jobs are running even slower with ccache enabled as long as the cache is not
>>>>>> populated yet. You only get a speedup afterwards. So if you add this now
>>>>>> without also adding the possibility to store the cache persistently, the
>>>>>> shared runners will try to populate the cache each time just to throw away
>>>>>> the results afterwards again. Thus all the shared runners only get slower
>>>>>> without any real benefit here.
>>>>>>
>>>>>> Thus we either need to get ccache working properly for the shared runners
>>>>>> first, or you have to think of a different way of enabling ccache for the
>>>>>> non-shared runners, so that it does not affect the shared runners
>>>>>> negatively.
>>>>>
>>>>> Is there anything functional holding up your previous full cccache support
>>>>> series from last month ? Or is it just lack of reviews ?
>>>>
>>>> It's basically the problems mentioned in the cover letter and Stefan's
>>>> comment here:
>>>>
>>>>  https://lists.gnu.org/archive/html/qemu-devel/2021-04/msg02219.html
>>>
>>> I'm not sure I understand why Stefan thinks gitlab's caching doesn't
>>> benefit ccache. We add ccache for libvirt GitLab CI, and AFAIR it
>>> sped up our builds significantly.
>>
>> I think Stefan is referring to a comment I made, when using both
>> shared runners and dedicated runners (what I'm currently testing)
>> various jobs are stuck transferring artifacts/cache {FROM, TO}
>> {shared, dedicated} runners at the same time, which is sub-optimal
>> because it saturate the dedicated runner network link.
> 
> I think we're over thinking things a bit too much and worrying about
> scenarios that we're not actually hitting that frequently today, and
> delaying the benefit for everyone.
> 
> Our common case is that most contributors are simply using shared
> runners exclusively, as is the main qemu repo staging branch. AFAIK
> these should benefit from a simple ccache enablement today.
> 
> Since there are questions about other setups though, we can just
> provide an easy way to turn it off. eg:
> 
>   if test -z "$QEMU_CI_SKIP_CCACHE"
>   then
>      PATH=/usr/local/libexec/ccache:$PATH
>   fi
> 
> anyone who wishes to disable it, can just set that variable in their
> git repo fork. If there are specific jobs we want to disable cccache
> for, those jobs can set that too.

OK, understood. I'll see with Willian how to have ccache working.

Thanks for the feedback,

Phil.