another error with md malloc based fs

[Chuck Swiger]

On Mar 22, 2007, at 4:17 PM, Michael Schuh wrote:
> i can't understand how malloc can eat all available
> memory, i have 2Gigs of it ;-)
> so it seems to me i know what i doing, if
> i have 1,6 Gigs free Memory, and i say ok get me 750Megs from
> my 1,6 Gigs of free Memory, what was faulty on that????

The two choices involve a swap-based RAMdisk, which can use all of  
the available physical RAM it needs to, since this memory is  
swappable, or a kernel-memory-based RAMdisk, which uses wired-down  
memory from within the kernel.

You shouldn't or can't create huge kernel-based RAMdisks without  
expanding the amount of KVM available to the kernel...it's not  
reasonable to wire down hundreds of MB or RAM in almost all situations.

> i hope it gave another way to use such ram-disk without using
> swap or other harddisk based backend....otoh my ram is not fully  
> useful
> for me and i must, after twelve years of using freebsd, looking  
> back to
> linux, and that i never would, only for using ramdisks....

If you wanted to create a 10MB RAMdisk, using kernel-based memory  
would be fine.

For things which are much larger than that, the kernel's global VM  
management is using techniques like page-fault frequency heuristics  
and will do a better job of keeping active stuff cached in RAM  
(regardless of whether the stuff is part of a running process or part  
of your RAMdisk filesystem), if you let the kernel manage the  
situation than if you try to configure hundreds of MB which can only  
be used for a RAMdisk.

Anyway, if you want to use Linux instead, nobody here is going to try  
to persuade you to do otherwise.  Have fun,

-- 
-Chuck