On Tue, Feb 15, 2022 at 9:03 PM <kerneljasonxing@xxxxxxxxx> wrote: > > From: Jason Xing <xingwanli@xxxxxxxxxxxx> > > Normally, user doesn't care the logic behind the kernel if they're > trying to set receive buffer via setsockopt. However, once the new > value of the receive buffer is set even though it's not smaller than > the initial value which is sysctl_tcp_rmem[1] implemented in > tcp_rcv_space_adjust(),, the server's wscale will shrink and then > lead to the bad bandwidth as intended. Quite confusing changelog, honestly. Users of SO_RCVBUF specifically told the kernel : I want to use _this_ buffer size, I do not want the kernel to decide for me. Also, I think your changelog does not really explain that _if_ you set SO_RCVBUF to a small value before connect() or in general the 3WHS, the chosen wscale will be small, and this won't allow future 10x increase of the effective RWIN. > > For now, introducing a new socket option to let the receive buffer > grow automatically no matter what the new value is can solve > the bad bandwidth issue meanwhile it's not breaking the application > with SO_RCVBUF option set. > > Here are some numbers: > $ sysctl -a | grep rmem > net.core.rmem_default = 212992 > net.core.rmem_max = 40880000 > net.ipv4.tcp_rmem = 4096 425984 40880000 > > Case 1 > on the server side > # iperf -s -p 5201 > on the client side > # iperf -c [client ip] -p 5201 > It turns out that the bandwidth is 9.34 Gbits/sec while the wscale of > server side is 10. It's good. > > Case 2 > on the server side > #iperf -s -p 5201 -w 425984 > on the client side > # iperf -c [client ip] -p 5201 > It turns out that the bandwidth is reduced to 2.73 Gbits/sec while the > wcale is 2, even though the receive buffer is not changed at all at the > very beginning. > > After this patch is applied, the bandwidth of case 2 is recovered to > 9.34 Gbits/sec as expected at the cost of consuming more memory per > socket. How does your patch allow wscale to increase after flow is established ? I would remove from the changelog these experimental numbers that look quite wrong, maybe copy/pasted from your prior version. Instead I would describe why an application might want to clear the 'receive buffer size is locked' socket attribute. > > Signed-off-by: Jason Xing <xingwanli@xxxxxxxxxxxx> > -- > v2: suggested by Eric > - introduce new socket option instead of breaking the logic in SO_RCVBUF > - Adjust the title and description of this patch > link: https://lore.kernel.org/lkml/CANn89iL8vOUOH9bZaiA-cKcms+PotuKCxv7LpVx3RF0dDDSnmg@xxxxxxxxxxxxxx/ > --- > I think adding another parallel SO_RCVBUF option is not good. It is adding confusion (and net/core/filter.c has been unchanged) Also we want CRIU to work correctly. So if you have a SO_XXXX setsockopt() call, you also need to provide getsockopt() implementation. I would suggest an option to clear or set SOCK_RCVBUF_LOCK, and getsockopt() would return if the bit is currently set or not. Something clearly describing the intent, like SO_RCVBUF_LOCK maybe.
