[openbeosnetteam] Re: mbuf's
- From: "Axel Dörfler" <axeld@xxxxxxxxxxxxxxxx>
- To: openbeosnetteam@xxxxxxxxxxxxx
- Date: Fri, 08 Feb 2002 17:14:24 CET (+0100)
> As an additional twist I've tried to implement a simple pool type
> allocator
> for fixed size blocks. basically this creates an area and then
> assigns bits
> of it to the requests for data or stores freed bits on a free block
> list.
> This should be much quicker than malloc/free and speed will be of the
> essence in the mbuf code.
> Does that answer peoples questions=3F
In addition, for outgoing packets it would make sense leave space at
the beginning of a buffer to prevent memmove()s.
When we have buffers of 2048 bytes anyway (and that's a good size IMHO)
we could have a small structure at the beginning which manages free
space in that block (simply start and end offsets).
I haven't looked into Gigabit ethernet etc. - does the packet size stay
the same for them=3F
> Well, there is one big question remaining, where do we switch from
> flat
> storage to mbufs=3F the logical place is in the driver for the card,
> but
> we're not planning on touching them straight away, so more reasonably
> it
> will partly depend on how we interface with the cards. I'd envisage
> at
> present it'll be done in an "if" layer that sits between the
> encapsulation
> and the network card.
There should definitely be a layer between the card driver and the rest
of the stack - that way, we can change the underlaying driver structure
to directly support mbufs.
I have also thought about a zero-copying network stack - this isn't
possible with the standard posix API (as the internal buffers have to
be copied into/back from the buffers provided by the read/write
function), but it would be possible to let the C++ Net API directly
pass the internal buffers to the application (could be a special mode).
The drawback is that the buffers may stay for a long time in user-land,
and we'd have to allocate new ones (enlarge our pool) - which could be
a possible stability problem.
Adios...
Axel.
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