Disabling nagle algorithm

[Peter Teoh]

On Mon, May 30, 2011 at 4:07 PM, C K Kashyap <ckkashyap at gmail.com> wrote:
>> Yes, TCP_NODELAY is a socket option:
>>
>> Going back to the kernel source:
>>
>> include/linux/tcp.h:
>>
>> #define TCP_NODELAY             1       /* Turn off Nagle's algorithm. */
>>
>> and this:
>>
>>       u8      nonagle     : 4,/* Disable Nagle algorithm?             */
>>
>> and looking into the kernel source code:
>>
>> ./fs/ocfs2/cluster/tcp.c:
>>        ret = sock->ops->setsockopt(sock, SOL_TCP, TCP_NODELAY,
>>
>> ./net/rds/tcp.c:
>>        sock->ops->setsockopt(sock, SOL_TCP, TCP_NODELAY, (char __user
>> *)&val,
>> the above are just two of the examples of how to set TCP_NODELAY in the
>> kernel.
>>
>> At the userspace level:
>>
>>
>> http://www.linuxquestions.org/questions/other-*nix-55/how-do-i-disable-the-nagle-algorithm-in-enterprise-linux-3-kernel-ver-2-4-21-4-a-556170/
>>
>> should be the correct way, except that the poster is attempting to do
>> it in a kernel that does not support it.
>>
>> As documented in http://linux.die.net/man/7/tcp, the supported kernel
>> is 2.5.71 and onwards.
>>
>
>  Okay ... so, the situation I have at hand is that there is this app I
> cannot modify. I'd like to see its performance with nagle disabled. Is this
> possible?

as u can see from the above two kernel implementation - each is
implementing the algorithm per-socket level, or connection-level, on
top of TCP, so, yes, u want to do that is possible.   but u will need
to write a kernel module, that implement itself as a netfilter module,
specifically just for the port that your application is using, and
setup socket connection of TCP_NODELAY type.   not sure if it will
work?

> While at it, is there a way to see the socket options of the socket
> descriptors in a running app?
> Regards,

Yes, i think so, just sniff out the network packet, and identify the
correct bit as according to the header:  include/linux/tcp.h:

<snip>
        u16     advmss;         /* Advertised MSS                       */
        u8      frto_counter;   /* Number of new acks after RTO */
        u8      nonagle     : 4,/* Disable Nagle algorithm?             */
                thin_lto    : 1,/* Use linear timeouts for thin streams */
                thin_dupack : 1,/* Fast retransmit on first dupack      */
                unused      : 2;
<snip>

> Kashyap



-- 
Regards,
Peter Teoh