Re: [uml-devel] mlock discussion is back - was: Re: [uml-user] odd ping problems

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

Alle 01:23, giovedì 27 maggio 2004, roland ha scritto:
> for at least two of the uml-pid`s you should see minor and major pagefaults
> (minflt/majflt) - i would say there is a releationship between those
> pagefaults and the ping delays. (i had turned off swap completely - so my
> host was not able to swap at all!)

That is not very good - if the latency problem comes from UML being swapped 
out, why do you disable swapping? Actually there is some "swapping", in a 
certain sense, so you could observe the major page faults (major means 
"requiring I/O", minor means "no IO": this can happen for COW pages, to mark 
as dirty a clean page).

When swapping is disabled, the pages belonging to a memory mapping, that was 
not modified since they were read from the disk, can be swapped out.

I.e. the executable pages, actually, are created by mmap'ing the executable; 
since they are not modified, they can be removed from memory.

Also the data pages which come from the executable can be treated the same 
way, if they are still "clean" (i.e. not modified).

[ I actually checked the code works this way, i.e. if you try to read a global 
var, even if it's mmaped with PROT_WRITE, in the page-tables it is marked as 
read-only; when you try to write it, it will be marked as read-write and the 
page will become dirty. ]

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



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