netdev - Re: iproute uses too small of a receive buffer

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Date:	Wed, 28 Oct 2009 20:50:48 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Ben Greear <greearb@...delatech.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	NetDev <netdev@...r.kernel.org>
Subject: Re: iproute uses too small of a receive buffer

Ben Greear wrote:
> On 10/28/2009 12:05 PM, Patrick McHardy wrote:
>> Eric Dumazet wrote:
>>> Stephen Hemminger a écrit :
>>>> Just having larger buffer isn't guarantee of success. Allocating
>>>> a huge buffer is not going to work on embedded.
>>>>
>>>
>>> Please note we do not allocate a big buffer, only allow more small skbs
>>> to be queued on socket receive queue.
>>>
>>> If memory is not available, skb allocation will eventually fail
>>> and be reported as well, embedded or not.
>>>
>>> I vote for allowing 1024*1024 bytes instead of 32768,
>>> and eventually user should be warned that it is capped by
>>> /proc/sys/net/core/rmem_max
>>
>> How about this? It will double the receive queue limit on ENOBUFS
>> up to 1024 * 1024b, then bail out with the normal error message on
>> further ENOBUFS.
>>
>> Signed-off-by: Patrick McHardy<kaber@...sh.net>
> 
> First:  This still pretty much guarantees that messages will be lost when
> the program starts (when messages are coming in too large of chunks for
> small buffers)
> If you are debugging something tricky, having lost messages will be
> very annoying!

Yeah, on second thought the probing also doesn't make too much sense
since the memory is only used when its really needed anyways. And its
capped by rmem_max.

> Second:  Why bail on ENOBUFS at all?  I don't see how it helps the user
> since they will probably just have to start it again, and will miss more
> messages than keeping going would have.

Agreed.

> And, even 1MB may not be enough for some scenarios.  So, probably best to
> let users over-ride the initial setting on cmd-line.  If not, then use
> a large value to start with.

How about this? It uses 1MB as receive buf limit by default (without
increasing /proc/sys/net/core/rmem_max it will be limited by less
however) and allows to specify the size manually using "-rcvbuf X"
(-r is already used, so you need to specify at least -rc).

Additionally rtnl_listen() continues on ENOBUFS after printing the
error message.

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