Re: [DISCUSSION] proposed mctl() API

[Matthew Wilcox <willy@infradead.org>]

On Thu, May 29, 2025 at 05:14:23PM -0400, Johannes Weiner wrote:
> 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.

Sorry, I must've missed that one ;-(

> 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.

So what about distinguishing between exited-normally processes (ie git
log) vs killed-by-oom processes (ie Barry's usecase)?  Update the
referenced bit in the first case and not the second?