Re: [patch] SLQB slab allocator

[Zhang, Yanmin]

On Tue, 2009-02-03 at 12:18 +0000, Hugh Dickins wrote:
> On Tue, 3 Feb 2009, Zhang, Yanmin wrote:
> > On Mon, 2009-02-02 at 11:00 +0200, Pekka Enberg wrote:
> > > On Mon, 2009-02-02 at 11:38 +0800, Zhang, Yanmin wrote:
> > > > Can we add a checking about free memory page number/percentage in function
> > > > allocate_slab that we can bypass the first try of alloc_pages when memory
> > > > is hungry?
> > > 
> > > If the check isn't too expensive, I don't any reason not to. How would
> > > you go about checking how much free pages there are, though? Is there
> > > something in the page allocator that we can use for this?
> > 
> > We can use nr_free_pages(), totalram_pages and hugetlb_total_pages(). Below
> > patch is a try. I tested it with hackbench and tbench on my stoakley
> > (2 qual-core processors) and tigerton (4 qual-core processors).
> > There is almost no regression.
> 
> May I repeat what I said yesterday?  Certainly I'm oversimplifying,
> but if I'm plain wrong, please correct me.
I did read your previous email carefully.

> 
> Having lots of free memory is a temporary accident following process
> exit (when lots of anonymous memory has suddenly been freed), before
> it has been put to use for page cache. 
Some workloads do something like what you said. But lots of workloads
don't use up most memory. Keepping an amount of free memory could
quicken up system running.

> The kernel tries to run with
> a certain amount of free memory in reserve, and the rest of memory
> put to (potentially) good use.  I don't think we have the number
> you're looking for there, though perhaps some approximation could
> be devised (or I'm looking at the problem the wrong way round).
> 
> Perhaps feedback from vmscan.c, on how much it's having to write back,
> would provide a good clue.  There's plenty of stats maintained there.
My startpoint is to look for a simple formula and the patch is a RFC.
If we consider statistics of write back (and so on), what I am worried about
is that page allocation called by allocate_slab might cause too many
try_to_free_pages/page reclaim/swap when memory is hungry. Originally, I thought
that's the root cause of your issue.

In other hand, free memory wouldn't vary too much with a good stably
running system. So the patch tries to change allocate_slab:
If there are high-order free pages available without reclaim/swap/free_pages,
just go to the first alloc_slab_page; If not, just jump to the second try of
alloc_slab_page. Of course, the checking is just a guess, not a guarantee.


> 
> > 
> > Besides this patch, I have another patch to try to reduce the calculation
> > of "totalram_pages - hugetlb_total_pages()", but it touches many files.
> > So just post the first simple patch here for review.
> > 
> > 
> > Hugh,
> > 
> > Would you like to test it on your machines?
> 
> Indeed I shall, starting in a few hours when I've finished with trying
> the script I promised yesterday to send you.  And I won't be at all
> surprised if your patch eliminates my worst cases, because I don't
> expect to have any significant amount of free memory during my testing,
> and my swap testing suffers from slub's thirst for higher orders.
> 
> But I don't believe the kind of check you're making is appropriate,
> and I do believe that when you try more extensive testing, you'll find
> regressions in other tests which were relying on the higher orders.
Yes, I agree. And we need find such tests which causes both memory used up
and lots of higher-order allocations.

> If all of your testing happens to have lots of free memory around,
> I'm surprised; but perhaps I'm naive about how things actually work,
> especially on the larger machines.
I use dozeons of benchmarks, but they are mostly microbench. In addition, mostly,
there are lots of free memory when they (except fio and membench) are running.

I tried to enable some big benchmarks, such like specweb2005, but I failed because
Tomcat scalability looks not good on 4 qual-core processor machines.

I would find and add new benchmark/tests which use up memory even cause lots of swap
into my testing framework.

> 
> Or maybe your tests are relying crucially on the slabs allocated at
> system startup, when of course there should be plenty of free memory
> around.
> 
> By the way, when I went to remind myself of what nr_free_pages()
> actually does, my grep immediately hit this remark in mm/mmap.c:
> 		 * nr_free_pages() is very expensive on large systems,
> I hope that's just a stale comment from before it was converted
> to global_page_state(NR_FREE_PAGES)!
I think so.

I would try to reproduce your issue on my machines.

Thanks for your scripts and suggestions.


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