Re: [TCP]: rcvbuf lock when tcp_moderate_rcvbuf enabled

Re: [TCP]: rcvbuf lock when tcp_moderate_rcvbuf enabled

[Dan Aloni]

Hello,

Below, I've forwarded change from 2.6.16 which I think may causes 
problems for applications that use setsockopt with SO_RCVBUF. We are 
using an implementation of an iSCSI target and according to network 
sniffs it seems that during data transfer the receive window 
unjustifyingly shrinks to a very low size (180 bytes). I can guess
that the code below indirectly affects the receive window size, but 
I'm not sure how it the logic works here, a clarification could be
helpful.

It's worth to mention that we have sysctl_tcp_moderate_rcvbuf=1, but 
I don't think it should interfere with applications that request to 
have a fixed receive buffer by the means of setsockopt(). I can also 
tell by experiment that reverting the change below makes the problem 
go away.

--- a97ed5416c28ee14ecab0ac4483c079a0c3e4c1d
+++ e9a54ae7d6903845598db14a8e1cba54026faf1b
@@ -456,7 +456,8 @@ void tcp_rcv_space_adjust(struct sock *s
 
 		tp->rcvq_space.space = space;
 
-		if (sysctl_tcp_moderate_rcvbuf) {
+		if (sysctl_tcp_moderate_rcvbuf &&
+		    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
 			int new_clamp = space;
 
 			/* Receive space grows, normalize in order to

-- 
Dan Aloni
da-x@monatomic.org, da-x@colinux.org, da-x@gmx.net, dan@xiv.co.il

Re: [TCP]: rcvbuf lock when tcp_moderate_rcvbuf enabled

[John Heffner]

On Thursday 23 March 2006 04:04, Dan Aloni wrote:
> Hello,
>
> Below, I've forwarded change from 2.6.16 which I think may causes
> problems for applications that use setsockopt with SO_RCVBUF. We are
> using an implementation of an iSCSI target and according to network
> sniffs it seems that during data transfer the receive window
> unjustifyingly shrinks to a very low size (180 bytes). I can guess
> that the code below indirectly affects the receive window size, but
> I'm not sure how it the logic works here, a clarification could be
> helpful.
>
> It's worth to mention that we have sysctl_tcp_moderate_rcvbuf=1, but
> I don't think it should interfere with applications that request to
> have a fixed receive buffer by the means of setsockopt(). I can also
> tell by experiment that reverting the change below makes the problem
> go away.

It shouldn't, but it used to.  That's exactly what this patch changes (making 
tcp_moderate_rcvbuf *not* override the application's requested buffer size).  
Is it possible the application isn't asking for enough buffer space, and that 
the kernel was just automatically helping it before this patch went in?

  -John