[ccache] Problem with using ccache during kernel compilation

Christopher Tate ctate at google.com
Tue Jan 21 11:08:57 MST 2014

If removing the .o files and recompiling results in pulling the .o files
out of ccache, but reconfiguring and recompiling does *not*, that implies
that the configuration step is literally producing some textually different
source file contents each time.

Does the kernel configuration include e.g. a generated string containing
the timestamp of when the configuration was performed, or some such thing?
 That sort of thing can easily force a full recompilation (since it would
mean that after each reconfiguration, even with the same settings and no
other source changes, the text of each compilation module would indeed be


On Tue, Jan 21, 2014 at 4:15 AM, Andrew Stubbs <ams at codesourcery.com> wrote:

> On 15/01/14 16:16, Piotr Kiszka wrote:
> > I am trying to use ccache during kernel building.But there is always
> > zero cache hits during second kernel compilation:
> I've observed this also. It's annoying!
> The problem seems to be something in the way the kernel config
> autogenerates the header files. I did find out what it was, but I've
> forgotten now.
> In fact, if you build the kernel, then delete all the .o files, and then
> run make again, you *do* get cache hits. "make clean" might be OK too, I'm
> not sure. Only if you start from completely clean, or reconfigure, then it
> doesn't work. Of course, this is the usual use case.
> I don't recall finding a workable solution.
> Sorry.
> Andrew
> P.s. A nice feature for ccache would be for it to report *why* a cached
> result wasn't found. This would only be meaningful in direct-mode, but
> still might be helpful. In preprocessor mode, *I* might find a list of
> hashes useful, so I could compare between runs, but this wouldn't be
> universally helpful, I suspect.
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