linux-kernel - Re: vmalloc performance

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Date:	Thu, 15 Apr 2010 01:35:48 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Nick Piggin <npiggin@...e.de>
Subject: Re: vmalloc performance

On Thu, 2010-04-15 at 00:13 +0900, Minchan Kim wrote:
> Cced Nick.
> He's Mr. Vmalloc.
> 
> On Wed, Apr 14, 2010 at 9:49 PM, Steven Whitehouse <swhiteho@...hat.com> wrote:
> >
> > Since this didn't attract much interest the first time around, and at
> > the risk of appearing to be talking to myself, here is the patch from
> > the bugzilla to better illustrate the issue:
> >
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index ae00746..63c8178 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -605,8 +605,7 @@ static void free_unmap_vmap_area_noflush(struct
> > vmap_area *va)
> >  {
> >        va->flags |= VM_LAZY_FREE;
> >        atomic_add((va->va_end - va->va_start) >> PAGE_SHIFT, &vmap_lazy_nr);
> > -       if (unlikely(atomic_read(&vmap_lazy_nr) > lazy_max_pages()))
> > -               try_purge_vmap_area_lazy();
> > +       try_purge_vmap_area_lazy();
> >  }
> >
> >  /*
> >
> >
> > Steve.
> >
> > On Mon, 2010-04-12 at 17:27 +0100, Steven Whitehouse wrote:
> >> Hi,
> >>
> >> I've noticed that vmalloc seems to be rather slow. I wrote a test kernel
> >> module to track down what was going wrong. The kernel module does one
> >> million vmalloc/touch mem/vfree in a loop and prints out how long it
> >> takes.
> >>
> >> The source of the test kernel module can be found as an attachment to
> >> this bz: https://bugzilla.redhat.com/show_bug.cgi?id=581459
> >>
> >> When this module is run on my x86_64, 8 core, 12 Gb machine, then on an
> >> otherwise idle system I get the following results:
> >>
> >> vmalloc took 148798983 us
> >> vmalloc took 151664529 us
> >> vmalloc took 152416398 us
> >> vmalloc took 151837733 us
> >>
> >> After applying the two line patch (see the same bz) which disabled the
> >> delayed removal of the structures, which appears to be intended to
> >> improve performance in the smp case by reducing TLB flushes across cpus,
> >> I get the following results:
> >>
> >> vmalloc took 15363634 us
> >> vmalloc took 15358026 us
> >> vmalloc took 15240955 us
> >> vmalloc took 15402302 us


> >>
> >> So thats a speed up of around 10x, which isn't too bad. The question is
> >> whether it is possible to come to a compromise where it is possible to
> >> retain the benefits of the delayed TLB flushing code, but reduce the
> >> overhead for other users. My two line patch basically disables the delay
> >> by forcing a removal on each and every vfree.
> >>
> >> What is the correct way to fix this I wonder?
> >>
> >> Steve.
> >>

In my case(2 core, mem 2G system), 50300661 vs 11569357. 
It improves 4 times. 

It would result from larger number of lazy_max_pages.
It would prevent many vmap_area freed.
So alloc_vmap_area takes long time to find new vmap_area. (ie, lookup
rbtree)

How about calling purge_vmap_area_lazy at the middle of loop in
alloc_vmap_area if rbtree lookup were long?

BTW, Steve. Is is real issue or some test?
I doubt such vmalloc bomb workload is real. 


-- 
Kind regards,
Minchan Kim



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