Re: [PATCH v2 06/13] fork: Add generic vmalloced stack support

[Michal Hocko]

On Mon 20-06-16 09:13:55, Andy Lutomirski wrote:
> On Mon, Jun 20, 2016 at 6:36 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > On Fri 17-06-16 13:00:42, Andy Lutomirski wrote:
> >> If CONFIG_VMAP_STACK is selected, kernel stacks are allocated with
> >> vmalloc_node.
> >
> > I like this! It also reduces demand for higher order (order-2) pages
> > considerably which is a great plus on its own. I would be little bit
> > worried about the performance because vmalloc wasn't the fastest one
> > AFAIR. Have you tried to measure that?
> 
> It seems to add about 1.5µs to pthread_create+join on my laptop.  (On
> an unmodified, stripped-down kernel, it took about 7µs before.  On a
> Fedora system, the baseline is much worse.)  I think that most of the
> overhead is because vmalloc allocates one page at a time, which means
> that it won't use a higher order page even if one is sitting on a
> freelist.

I guess a less artificial test case which would would generate a lot of
tasks and some memory pressure would be more representative (e.g.
kernbench). The thing is that even order-2 pages might get quite
expensive when the memory is fragmented.

> I can imagine better integration with the page allocator in which
> higher order pages are used if readily available.  Similarly, vfree
> could free pages that happen to be aligned and consecutive as a unit
> to avoid the overhead of merging them back together one at a time.
> 
> But I'm not planning on doing any of this myself any time soon.  I
> just want to get the code working and merged.

I agree, there is a room for improvement but no necessarily as a part of
this series.

-- 
Michal Hocko
SUSE Labs