Re: [cmake] Problem with dependencies for configure checks

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-06-04 14:54:31

Niall Douglas wrote:
> On 04/06/2021 13:04, Andrey Semashev via Boost wrote:
> > I could add a runtime
> > check, but I definitely don't want that check to happen on every call.
> > This means the result must be stored globally in a thread-safe manner.
> 
> At work we've got llfio::stat_t::fill() being called in some places millions of
> times per second.
> 
> It is implemented by attempting statx first, and if it is not supported, then stat
> or other syscalls to emulate the same.
> 
> You cannot cache statx support/non-support because only some filing systems
> support statx, and others do not. So depending on the path used, it may work,
> but at other times, not.
> 
> Right now at work we're on kernels which don't support statx, so we are
> always getting back ENOSYS and then falling back to emulation. We have not
> found any performance issues.

A kernel call is infinitely more expensive than checking a number against
ENOSYS. It's absolutely impossible for the test to ever be a performance issue.