Subject: Re: [e1000 2.6 10/11] TxDescriptors -> 1024 default

From: Ben Greear <[email-masked]>
Date: Fri, 12 Sep 2003 11:12:32 -0700

jamal wrote:
> On Fri, 2003-09-12 at 01:13, Feldman, Scott wrote:

>>On the other hand, if we're always running the descriptor ring near
>>empty, we've got other problems.  It seems to reason that it doesn't
>>matter how big the ring is if we're in that situation.  If the CPU can
>>overrun the device, expanding the queues between the CPU and the device
>>may help with bursts but gets you nothing for a sustained load.
>>
> 
> 
> Well, theres only one way out that device ;-> and it goes out at a max 
> rate of Gige. If you have sustained incoming rates from the CPU(s) of
> greater than Gige, then you are fucked anyways and you are better to
> drop at the scheduler queue.

I have seen greater packets-per-second throughput when I increase the TxDescriptor
ring (and RxDescriptor ring) when using pktgen, which checks for enqueue errors
and re-queues as needed.  So, it could help the case where we are running
at very high sustained speeds (or high packets-per-second rates).

>>I flunked that queuing theory class anyway, so what do I know?  Every
>>time I get stuck in a traffic slug on the freeway, I think about that
>>class.  Hey, that means my car is like an skb, so maybe longer roads
>>would help?  Not!
> 
> 
> 
> Note we do return an indication that the packet was dropped. What you do
> with that information is relative. TCP makes use of it in the kernel
> which makes sense. UDP congestion control is mostly under the influence
> of the UDP app in user space. The impedance between user space and
> kernel makes that info useless to the UDP app especially in cases when
> the system is overloaded (which is where this matters most). This is of
> course theory and someone who really wants to find out should
> experiment. I would be pleasantly shocked if it turned out the info to
> the UDP app was useful. An interesting thing to try , which violates
> UDP, is to have UDP requeue a packet back to the socket queue in the
> kernel everytime an indication is received that the scheduler queue
> dropped the packet. User space by virtue of UDP sock queue not emptying
> should find out soon and slow down.

Um, I doubt the UDP protocol says you MUST drop packets when you reach
congestion...it just says that you _CAN_ drop the packet.  Slowing
down user-space is exactly what you want to do in this case because it
saves user-space CPU, and it saves the user-space program from having
to deal (so often) with dropped packets.

Already, if the socket queue is full, poll/select will block, you'll get
-EBUSY returned, and/or your application will block on a wait queue....
Any of these allow the user space program to immediately back off,
saving the whole system work.

> All this is really speculation:
> A UDP app that really care about congestion should factor it from an end
> to end perspective and use the big socket queues suggested to buffer
> things.

Big socket queues can cause your machine to over-run the scheduler queue,
if I understand Dave right.  And lots of big queues everywhere can cause
your machine to OOM and lock up completely (see another recent thread).

> 
> To give anology to your car, if you only find out half way later that
> there was a red light a few meters back then that info is useless. If
> you dont get hit and reverse you may find that infact the light has
> turned to green which is again useless ;-> 

So much better to have stopped the car earlier and kept him out of the
intersection in the first place :)

Ben

-- 
Ben Greear <[email-masked]>
Candela Technologies Inc  http://www.candelatech.com