linux-kernel - Re: [PATCH] mm: cache largest vma

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Message-ID: <20131104073640.GF13030@gmail.com>
Date:	Mon, 4 Nov 2013 08:36:40 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Guan Xuetao <gxt@...c.pku.edu.cn>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: cache largest vma


* Davidlohr Bueso <davidlohr@...com> wrote:

> I will look into doing the vma cache per thread instead of mm (I hadn't 
> really looked at the problem like this) as well as Ingo's suggestion on 
> the weighted LRU approach. However, having seen that we can cheaply and 
> easily reach around ~70% hit rate in a lot of workloads, makes me wonder 
> how good is good enough?

So I think it all really depends on the hit/miss cost difference. It makes 
little sense to add a more complex scheme if it washes out most of the 
benefits!

Also note the historic context: the _original_ mmap_cache, that I 
implemented 16 years ago, was a front-line cache to a linear list walk 
over all vmas (!).

This is the relevant 2.1.37pre1 code in include/linux/mm.h:

/* Look up the first VMA which satisfies  addr < vm_end,  NULL if none. */
static inline struct vm_area_struct * find_vma(struct mm_struct * mm, unsigned long addr)
{
        struct vm_area_struct *vma = NULL;

        if (mm) {
                /* Check the cache first. */
                vma = mm->mmap_cache;
                if(!vma || (vma->vm_end <= addr) || (vma->vm_start > addr)) {
                        vma = mm->mmap;
                        while(vma && vma->vm_end <= addr)
                                vma = vma->vm_next;
                        mm->mmap_cache = vma;
                }
        }
        return vma;
}

See that vma->vm_next iteration? It was awful - but back then most of us 
had at most a couple of megs of RAM with just a few vmas. No RAM, no SMP, 
no worries - the mm was really simple back then.

Today we have the vma rbtree, which is self-balancing and a lot faster 
than your typical linear list walk search ;-)

So I'd _really_ suggest to first examine the assumptions behind the cache, 
it being named 'cache' and it having a hit rate does in itself not 
guarantee that it gives us any worthwile cost savings when put in front of 
an rbtree ...

Thanks,

	Ingo
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