Re: Note describing poor dcache utilization under high memory pressure

[Daniel Phillips]

On January 30, 2002 10:07 am, Horst von Brand wrote:
> Daniel Phillips <[e-mail]> said:
> > On January 29, 2002 12:54 pm, Helge Hafting wrote:
> > > Momchil Velikov wrote:
> 
> [...]
> 
> > > > Umm, all the ptes af the parent ought to be made COW, no ?
> 
> > > Sure. But quite a few of them may be COW already, if the parent 
> > > itself is a result of some earlier fork.
> 
> > Right, or if the parent has already forked at least one child.
> 
> But most of this will be lost on exec(2).

Even if we doing nothing more than the algorithm on the table, I doubt you'll 
see a measurable overhead on fork+exec.  Certainly it will be as good or 
better than what we have currently.

If that's not good enough, I'm considering keeping a bit on the page table 
indicating whether are particular page table is currently in the 'all CoWable 
ptes set RO' state, and if so, don't do it again.  I think that with this 
small optimization, the value of further improvements will be small indeed.

That said, Linus's suggestion of using the x86's ability to have the 
writeprotect bits in a page directory override the protections at the page 
level is a good one, and reduces the cost of detecting the fork+exec case to 
a very small number of faults - none if we are clever.  But this is entirely 
secondary to the main goal of sharing page tables at all, which is a rather 
fundamental shift in the way the Linux VM works.  (Though it seems the patch 
will be small.)

> Also, it is my impression that
> the tree of _running_ processes isn't usually very deep (Say init --> X -->
> [Random processes] --> [compilations &c], this would make 5 or 6 deep, no
> more.

Worst case is just as important as typical case here, since there will always 
be x% of users out there whose normal workload consists entirely of worst 
case.

> Should take a pstree(1) listing on a busy machine and work out some
> statistics... here (a personal worstation) the tree is very fat at the
> first level below init(8), and just 5 deep when running pstree(1)).

Here's my tree - on a non-very-busy laptop.  Why is my X tree so much deeper? 
I suppose if I was running java this would look considerably more interesting.

init-+-apache---8*[apache]
     |-apmd
     |-bash---bash---xinit-+-XFree86
     |                     `-xfwm-+-xfce---gnome-terminal-+-bash---pstree
     |                            |                       `-gnome-pty-helpe
     |                            `-xfgnome
     |-cardmgr
     |-cupsd---http
     |-5*[getty]
     |-gpm
     |-kapm-idled
     |-kdeinit---kdeinit
     |-5*[kdeinit]
     |-kdesud
     |-keventd
     |-kmail
     |-mozilla-bin---mozilla-bin---3*[mozilla-bin]
     |-portmap
     |-sshd
     `-xchat

> Sure, all processes will all end up sharing glibc, and the graphical stuff 
> will share the X &c libraries, so this would end up being a win this way.

Nobody has suggested that the sharing algorithm as described isn't a win, 
IMHO, we are quibbling over the last few percent of the win.  It's getting 
high time to end the suspense by benchmarking the code.

Caveat: the page table sharing as described does not do a lot for shared 
mmaps, such as glibc.  (Unless those are inherited through a fork of course, 
then it helps a lot.)  Let me reiterate my goal with this patch: *Fix The 
Fork Problem With Rmap* so that we can quit spending months fiddling with 
virtual scanning, trying to get it to work properly (it never will).

I see the value in the various suggestions I've received, but what I don't 
see is the value in delaying, or getting stuck adding new features.  Let's 
concentrate on making the simple thing I've described work *now* and add 
features to it later.

I'm gratified that nobody has yet pointed out any fundamental flaws that 
would keep it from working.  I wasn't at all sure of that when I set out on 
this path a month ago.

-- 
Daniel