[uml-devel] Re: InnoDB and /proc/mm

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

I've read your page at http://brad.livejournal.com/1968050.html, and was 
attracted by it since I'm studying Uml internals. However, since I've read a 
lot of times that patch, I can give you some suggestion about it.

What it does is simply to allow one process (let's call it the *kernel* 
process, referring to UML) to create more virtual address spaces for only one 
process (let's call it the userspace one). It accomplishes this by ptracing 
the child and making some special requests.

Not arbitrary ones (they obey all the rule that a normal address space has), 
as you say. On your page it seems that /proc/mm enables apps to create a 
36-bit address space; maybe I just don't get you right, but you say exactly 
this at least about Windows AWE API. Which is not right. Keep reading, 
however, as there is (maybe) a way to use it.

The reason for this is that the userspace process will have one address space 
when running pid 1 inside UML, another while running pid 2 inside UML and so 
on (the userspace thread just runs the code of processes inside UML). Those 
address space are also unrelated (they can decide to share pages, yes, by 
mmaping a common file, which can be on a tmpfs, i.e. in RAM; currently there 
is no other way of creating shared memory between them).

Then, with the SKAS patch you can say: well, I create 10 address spaces of 3 
Gigas, and I make one InnoDB process (I don't think one thread) switch back 
and forth between them, to keep the various datas. 30 Gigas total.
If you mean this, you are right. But that's *very* hard to do, since when you 
use one of the address spaces, you can't use the other one. So if you must 
use datas scattered between them, that's a real pain.
Also, I'm not that sure that the overhead for /proc/mm is so little that you 
get a performance gain. The "user" thread can't directly change its address 
space; it is the "kernel" thread which must do this (as of now; this maybe 
could be changed, if you find anyone able to do it).

Regards
-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729



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