Re: Bugs and wishes in memory management area

Kai M{kisara (makisara@abies.metla.fi)
Sat, 23 Nov 1996 13:35:03 +0200 (EET)

On Sat, 23 Nov 1996, Gerard Roudier wrote:
...
> In my flameable opinion, a usable system nowaday needs more than 16 MB, or
> at least 16 MB, regardless the O/S it is running.
>
I disagree. Mybe for software development and general computing but Linux
is very nice for more special applications. Examples:
- our airborne imaging spectrometer "host" computer doubling as "tape
server" when it is in our office between flights (8 MB)
- the laptop we use to navigate the flight lines (386 with 4 MB)
- my personal laptop (8 MB, running nicely emacs with X, at least I
prefer this over running Windows)

IMHO Linux should be useful to similar applications also in future.

...
> On the other hand, being given that 128/16=8, 87% IOs will need to be
> double-buffered. Doing DMA in that conditions is in my opinion pretty
> stupid.
>
It is instructive to compute how much double buffering really slows down
things. Let's assume a peripheral capable of 0.5 MB/s and memory to memory
transfer rate of 20 MB/s. With double buffering for all transfers the rate
is 0.488 MB/s (assuming double buffering is implemented properly). May be
stupid but in many cases it is practical ;-)

Besides, I don't think the issue here is ISA DMA, it is >allocation of a
largish chunk of memory that is contiguous in physical memory<. The ISA
DMA is special only in that the chunk must be below 16 MB. These kinds of
allocations are needed for peripherals that don't support scatter/gather
(or don't support enough scatter/gather segments). One example might be a
PCI video board that wants to transfer images to a memory buffer.

Kai