Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB

[Vito Caputo]

On Tue, Nov 16, 2021 at 11:55:41AM -0700, Jens Axboe wrote:
> On 11/16/21 11:36 AM, Matthew Wilcox wrote:
> > On Mon, Nov 15, 2021 at 08:35:30PM -0800, Andrew Morton wrote:
> >> I'd also be interested in seeing feedback from the MM developers.
> > [...]
> >> Subject: Increase default MLOCK_LIMIT to 8 MiB
> > 
> > On the one hand, processes can already allocate at least this much
> > memory that is non-swappable, just by doing things like opening a lot of
> > files (allocating struct file & fdtable), using a lot of address space
> > (allocating page tables), so I don't have a problem with it per se.
> > 
> > On the other hand, 64kB is available on anything larger than an IBM XT.
> > Linux will still boot on machines with 4MB of RAM (eg routers).  For
> > someone with a machine with only, say, 32MB of memory, this allows a
> > process to make a quarter of the memory unswappable, and maybe that's
> > not a good idea.  So perhaps this should scale over a certain range?
> > 
> > Is 8MB a generally useful amount of memory for an iouring user anyway?
> > If you're just playing with it, sure, but if you have, oh i don't know,
> > a database, don't you want to pin the entire cache and allow IO to the
> > whole thing?
> 
> 8MB is plenty for most casual use cases, which is exactly the ones that
> we want to "just work" without requiring weird system level
> modifications to increase the memlock limit.
> 

Considering a single fullscreen 32bpp 4K-resolution framebuffer is
~32MiB, I'm not convinced this is really correct in nearly 2022.

If we're going to bump the default at the kernel, I'm with Matthew on
making it autoscale within a sane range, depending on available
memory.

As an upper bound I'd probably look at the highest anticipated
consumer resolutions, and handle a couple fullscreen 32bpp instances
being pinned.

Regards,
Vito Caputo