Re: [PATCH 0/2] execve scalability issues, part 1

[Mateusz Guzik]

On Mon, Aug 21, 2023 at 02:07:28PM -0700, Dennis Zhou wrote:
> On Mon, Aug 21, 2023 at 10:28:27PM +0200, Mateusz Guzik wrote:
> > With this out of the way I'll be looking at some form of caching to
> > eliminate these allocs as a problem.
> > 
> 
> I'm not against caching, this is just my first thought. Caching will
> have an impact on the backing pages of percpu. All it takes is 1
> allocation on a page for the current allocator to pin n pages of memory.
> A few years ago percpu depopulation was implemented so that limits the
> amount of resident backing pages.
> 

I'm painfully aware.

> Maybe the right thing to do is preallocate pools of common sized
> allocations so that way they can be recycled such that we don't have to
> think too hard about fragmentation that can occur if we populate these
> pools over time?
> 

This is what I was going to suggest :)

FreeBSD has a per-cpu allocator which pretends to be the same as the
slab allocator, except handing out per-cpu bufs. So far it has sizes 4,
8, 16, 32 and 64 and you can act as if you are mallocing in that size.

Scales perfectly fine of course since it caches objs per-CPU, but there
is some waste and I have 0 idea how it compares to what Linux is doing
on that front.

I stress though that even if you were to carve out certain sizes, a
global lock to handle ops will still kill scalability.

Perhaps granularity better than global, but less than per-CPU would be a
sweet spot for scalabability vs memory waste.

That said...

> Also as you've pointed out, it wasn't just the percpu allocation being
> the bottleneck, but percpu_counter's global lock too for hotplug
> support. I'm hazarding a guess most use cases of percpu might have
> additional locking requirements too such as percpu_counter.
> 

True Fix(tm) is a longer story.

Maybe let's sort out this patchset first, whichever way. :)

> Thanks,
> Dennis
> 
> > Thoughts?
> > 
> > Mateusz Guzik (2):
> >   pcpcntr: add group allocation/free
> >   fork: group allocation of per-cpu counters for mm struct
> > 
> >  include/linux/percpu_counter.h | 19 ++++++++---
> >  kernel/fork.c                  | 13 ++------
> >  lib/percpu_counter.c           | 61 ++++++++++++++++++++++++----------
> >  3 files changed, 60 insertions(+), 33 deletions(-)
> > 
> > -- 
> > 2.39.2
> >