Re: [patch 00/14] remap_file_pages protection support

From: Blaisorblade
Date: Wed May 17 2006 - 02:14:36 EST

On Tuesday 16 May 2006 18:47, Andreas Mohr wrote:
> Hi,
> On Tue, May 16, 2006 at 09:31:12AM -0700, Valerie Henson wrote:
> > On Tue, May 16, 2006 at 03:51:35PM +0200, Andreas Mohr wrote:
> > > I cannot offer much other than some random confirmation that from my
> > > own oprofiling, whatever I did (often running a load test script
> > > consisting of launching 30 big apps at the same time), find_vma
> > > basically always showed up very prominently in the list of
> > > vmlinux-based code (always ranking within the top 4 or 5 kernel
> > > hotspots, such as timer interrupts, ACPI idle I/O etc.pp.).
> > > call-tracing showed it originating from mmap syscalls etc., and AFAIR
> > > quite some find_vma activity from oprofile itself.
> >
> > This is important: Which kernel?

I'd also add (for all peoples): on which processors? L2 cache size probably
plays an important role, if (as I'm convinced) the problem are cache misses
during rb-tree traversal.

> I had some traces still showing find_vma prominently during a profiling run
> just yesterday, with a very fresh 2.6.17-rc4-ck1 (IOW, basically
> 2.6.17-rc4). I added some cache prefetching in the list traversal a while
> ago,

You mean the rb-tree traversal, I guess! Or was the base kernel so old?

> and IIRC that improved profiling times there, but cache prefetching is
> very often a bandaid in search for a real solution: a better data-handling
> algorithm.

Ok, finally I find the time to kick in and ask a couple of question.

The current algorithm is good but has poor cache locality (IMHO).

First, since you can get find_vma on the profile, I've read (the article
talked about userspace apps but I think it applies to kernelspace too) that
oprofile can trace L2 cache misses.

I think such a profiling, if possible, would be particularly interesting:
there's no reason whatsoever for that lookup, even on a 32-level tree
(theoretical maximum since we have max 64K vmas and height_rbtree <= 2 logN),
should be so slow, unless you add cache misses into the picture. The fact
that cache prefetching helps shows this even more.

The lookup has very poor cache locality: the rb-node (3 pointers i.e. 12
bytes, and we need only 2 pointers on searches) is surrounded by non-relevant
data we fetch (we don't need the VMA itself for nodes we traverse).

For cache-locality the best data structure I know of are radix trees; but
changing the implementation is absolutely non-trivial (the find_vma_prev()
and friends API is tightly coupled with the rb-tree), and the size of the
tree grows with the virtual address space (which is a problem on 64-bit
archs); finally, you have locality when you do multiple searches, especially
for the root nodes, but not across different levels inside a single search.
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)

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