On Thu, May 29, 2025 at 04:28:46PM +0100, Matthew Wilcox wrote:
> Barry's problem is that we're all nervous about possibly regressing
> performance on some unknown workloads. Just try Barry's proposal, see
> if anyone actually compains or if we're just afraid of our own shadows.
I actually explained why I think this is a terrible idea. But okay, I
tried the patch anyway.
This is 'git log' on a hot kernel repo after a large IO stream:
VANILLA BARRY
Real time 49.93 ( +0.00%) 60.36 ( +20.48%)
User time 32.10 ( +0.00%) 32.09 ( -0.04%)
System time 14.41 ( +0.00%) 14.64 ( +1.50%)
pgmajfault 9227.00 ( +0.00%) 18390.00 ( +99.30%)
workingset_refault_file 184.00 ( +0.00%) 236899.00 (+127954.05%)
Clearly we can't generally ignore page cache hits just because the
mmaps() are intermittent.
The whole point is to cache across processes and their various
apertures into a common, long-lived filesystem space.
Barry knows something about the relationship between certain processes
and certain files that he could exploit with MADV_COLD-on-exit
semantics. But that's not something the kernel can safely assume. Not
without defeating the page cache for an entire class of file accesses.
