Re: [PATCH v31 05/13] mm/damon: Implement primitives for the virtual memory address spaces

[SeongJae Park]

From: SeongJae Park <sjpark@xxxxxxxxx>

On Thu, 24 Jun 2021 07:42:44 -0700 Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:

> On Thu, Jun 24, 2021 at 3:26 AM SeongJae Park <sj38.park@xxxxxxxxx> wrote:
> >
> [...]
> > > > +/*
> > > > + * Get the three regions in the given target (task)
> > > > + *
> > > > + * Returns 0 on success, negative error code otherwise.
> > > > + */
> > > > +static int damon_va_three_regions(struct damon_target *t,
> > > > +                               struct damon_addr_range regions[3])
> > > > +{
> > > > +       struct mm_struct *mm;
> > > > +       int rc;
> > > > +
> > > > +       mm = damon_get_mm(t);
> > > > +       if (!mm)
> > > > +               return -EINVAL;
> > > > +
> > > > +       mmap_read_lock(mm);
> > > > +       rc = __damon_va_three_regions(mm->mmap, regions);
> > > > +       mmap_read_unlock(mm);
> > >
> > > This is being called for each target every second by default. Seems
> > > too aggressive. Applications don't change their address space every
> > > second. I would recommend to default ctx->primitive_update_interval to
> > > a higher default value.
> >
> > Good point.  If there are many targets and each target has a huge number of
> > VMAs, the overhead could be high.  Nevertheless, I couldn't find the overhead
> > in my test setup.  Also, it seems someone are already started exploring DAMON
> > patchset with the default value. and usages from others.  Silently changing the
> > default value could distract such people.  So, if you think it's ok, I'd like
> > to change the default value only after someone finds the overhead from their
> > usages and asks a change.
> >
> > If you disagree or you found the overhead from your usage, please feel free to
> > let me know.
> >
> 
> mmap lock is a source contention in the real world workloads. We do
> observe in our fleet and many others (like Facebook) do complain on
> this issue. This is the whole motivation behind SFP, maple tree and
> many other mmap lock scalability work. I would be really careful to
> add another source of contention on mmap lock. Yes, the user can
> change this interval themselves but we should not burden them with
> this internal knowledge like "oh if you observe high mmap contention
> you may want to increase this specific interval". We should set a good
> default value to avoid such situations (most of the time).

Thank you for this nice clarification.  I can understand your concern because I
also worked for an HTM-based solution of the scalability issue before.

However, I have neither strong preference nor confidence for the new default
value at the moment.  Could you please recommend one if you have?


Thanks,
SeongJae Park