Re: [DISCUSSION] proposed mctl() API

[Johannes Weiner <hannes@cmpxchg.org>]

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.