[ale] Choke point (or when to bring on router)

[Bao C. Ha]

Hi Bob,

> >The Linux kernel seem to have problems pushing packets above 
> >OC-12 ATM/Sonnet speed (622 Mbits/s).  Many people choose to 
> >move over to FreeBSD at this point.  But I think it should be
> >fixed rather easily.

Oh oh!  My big mouth gets me into trouble again.  Take my
comments with salt since they may have been out-dated with
the speed of Linux's forward momentum.

> If one made the jump to FreeBSD, what would be the limiting 
> point?  Or, 
> what is the fix for Linux?

I don't know much about FreeBSD, except I like the cute leather-
clad red devil girl I saw at LinuxWorld two years ago.   FreeBSD 
4.x has some problems at high speed transfers few months ago, 
compared to 3.5.  It should have been fixed by now.  The 
limitation would be the PCI bus: about 8 gig/s for PCI-X(?).

As I recalled, from last summer, there are three problems with
Linux at gigabit networking level.

- Promiscous mode.  Some device drivers will suffer huge
performance loss when entering promiscous mode.  You don't need
promiscous mode unless you also want to do network monitoring.
And most people at this level will write their own driver.

- Double copy problem.  The networking stack used to copy from
wire/device to kernel, then kernel to user space.  The network
bandwidth depends on the speed of the processor.  With last
year's best processor, the limit is about OC-12.  Kernel 2.4
implements the zero-copy policy, I believe.

- VM.  I think it is bogus, since a Linux router will reach its
physical bandwidth limitation before it runs out of room.  Well?
Unless it is also doing layer 7 stuff like squid/content
filtering.

The main fix is to use the latest kernels: 2.4.17 or 2.4.18.

Thanks.
Bao


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