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.