Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups

Nick Piggin
2007-04-13T02:12:00Z

Matt Mackall wrote:
> On Fri, Apr 13, 2007 at 11:01:41AM +1000, Nick Piggin wrote:
>
>>>Basically: to show what the hell's going on in the VM.
>>
>>kprobes / systemtap isn't good enough?
>
>
> It's not really a good match to the kprobes model. I'm not interested
> in events, per se. I don't want to need to know about every single
> alloc/free of N different varieties integrated from boot onward to
> build up an image of the state of the system. Instead, I want to take
> snapshots of the state of the VM.

Systemtap can't output a large set of values?

Why can't you attach a kprobe to a dummy syscall, and from there
iterate over pgdat/zone/memmap and output what you want?

Actually I'm surprised that kind of data querying facility isn't
already in there (I haven't used it seriously though).

> The main goal here is to be able to answer the question "where's my
> memory going?". Currently you can't really give a good answer to that
> question from userspace because of shared mappings, etc.
>
> There are lots of secondary questions that follow on very quickly from
> that, like "what parts of my shared mappings are or aren't shared, and
> why?", "what's actually in my application's working set?" and "how much
> of this crap can I ditch?".

I understand roughly what you want, and that you can't easily get
it from /proc currently. My question at this point is just why can
we not use systemtap.

--
SUSE Labs, Novell Inc.