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be in ticks "for algorithm stability" when originally committed, it turns out that it has a significant impact in timing out connections. When we changed HZ from 100 to 1000, this had a big effect on reducing the time before dropping connections. To demonstrate, boot with kern.hz=100. ssh to a box on local ethernet and establish a reliable round-trip-time (ie: type a few commands). Then unplug the ethernet and press a key. Time how long it takes to drop the connection. The old behavior (with hz=100) caused the connection to typically drop between 90 and 110 seconds of getting no response. Now boot with kern.hz=1000 (default). The same test causes the ssh session to drop after just 9-10 seconds. This is a big deal on a wifi connection. With kern.hz=1000, change sysctl net.inet.tcp.rexmit_min from 3 to 30. Note how it behaves the same as when HZ was 100. Also, note that when booting with hz=100, net.inet.tcp.rexmit_min *used* to be 30. This commit changes TCPTV_MIN to be scaled with hz. rexmit_min should always be about 30. If you set hz to Really Slow(TM), there is a safety feature to prevent a value of 0 being used. This may be revised in the future, but for the time being, it restores the old, pre-hz=1000 behavior, which is significantly less annoying. As a workaround, to avoid rebooting or rebuilding a kernel, you can run "sysctl net.inet.tcp.rexmit_min=30" and add "net.inet.tcp.rexmit_min=30" to /etc/sysctl.conf. This is safe to run from 6.0 onwards. Approved by: re (rwatson) Reviewed by: andre, silby
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