linux-kernel - Re: [PATCH 0/13] Parallel struct page initialisation v4

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Date:	Tue, 5 May 2015 19:01:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Mel Gorman <mgorman@...e.de>, Nathan Zimmer <nzimmer@....com>,
	Dave Hansen <dave.hansen@...el.com>,
	Scott Norton <scott.norton@...com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/13] Parallel struct page initialisation v4

On Tue, 05 May 2015 21:21:19 -0400 Waiman Long <waiman.long@...com> wrote:

> On 05/05/2015 04:02 PM, Andrew Morton wrote:
> > On Tue, 5 May 2015 11:45:14 +0100 Mel Gorman<mgorman@...e.de>  wrote:
> >
> >> On Mon, May 04, 2015 at 02:30:46PM -0700, Andrew Morton wrote:
> >>>> Before the patch, the boot time from elilo prompt to ssh login was 694s.
> >>>> After the patch, the boot up time was 346s, a saving of 348s (about 50%).
> >>> Having to guesstimate the amount of memory which is needed for a
> >>> successful boot will be painful.  Any number we choose will be wrong
> >>> 99% of the time.
> >>>
> >>> If the kswapd threads have started, all we need to do is to wait: take
> >>> a little nap in the allocator's page==NULL slowpath.
> >>>
> >>> I'm not seeing any reason why we can't start kswapd much earlier -
> >>> right at the start of do_basic_setup()?
> >> It doesn't even have to be kswapd, it just should be a thread pinned to
> >> a done. The difficulty is that dealing with the system hashes means the
> >> initialisation has to happen before vfs_caches_init_early() when there is
> >> no scheduler.
> > I bet we can run vfs_caches_init_early() after sched_init().  Might
> > need a few little fixups.
> >
> >> Those allocations could be delayed further but then there is
> >> the possibility that the allocations would not be contiguous and they'd
> >> have to rely on CMA to make the attempt. That potentially alters the
> >> performance of the large system hashes at run time.
> > hm, why.  If the kswapd threads are running and busily creating free
> > pages then alloc_pages(order=10) can detect this situation and stall
> > for a while, waiting for kswapd to create an order-10 page.
> >
> > Alternatively, the page allocator can go off and synchronously
> > initialize some pageframes itself.  Keep doing that until the
> > allocation attempt succeeds.
> >
> > Such an approach is much more robust than trying to predict how much
> > memory will be needed.
> >
> 
> Most of those hash tables are allocated before smp_boot. In UP mode, you 
> can't have another thread initializing memory. So we really need to 
> preallocate enough for those tables.

(copy-paste)

: Alternatively, the page allocator can go off and synchronously
: initialize some pageframes itself.  Keep doing that until the
: allocation attempt succeeds.

IOW, the caller of alloc_pages() goes off and does the work which
kswapd would have done later on.

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