network performance over 1GBps links degraded

[Gary Jennejohn]

I have two computers, both with 1Gbps interfaces plugged into a
1Gb switch.  One computer is running FreeBSD HEAD and the other
some version of Linux.  Under FreeBSD I have re0 and under Linux
I don't know what the hardware is.

Both interfaces are using a MTU of 4088 because that's the
maximum my re0 supports.

I tend to copy files from one to the other using ftp fairly
frequently.

I noticed that the transfer speed has dropped to only about
12MBps.  I'm used to seeing about 27MBps during the ftp
transfers.

I also observed the drop in transfer speed between FreeBSD and
a Windows 10 computer.  Formerly, I was seeing about 30MBps.
Windows 10 also has MTU 4088 set.

I tested with a FreeBSD kernel from March 7 and one from March
17, but both show the miserable performance.  Unfortunately, I
don't have any older kernels backed up.

I can't say exactly when the performance degraded, but possibly
there was some change to the kernel on or before March 7 which
caused it.

Has anyone else seen this?

There were several changes to the networking stack lately.

I wonder whether anyone has an idea what could be the cause of the
performance degredation, and what sysctls I could set to get the
old performance back.

-- 
Gary Jennejohn