Re: Request for Testing: TCP RACK

I don't have the full context, but it seems like the complaint is a performance regression in bonnie++ and perhaps other things when tcp_hpts is loaded, even when it is not used.  Is that correct?

If so, I suspect its because we drive the tcp_hpts_softclock() routine from userret(), in order to avoid tons of timer interrupts and context switches.  To test this theory,  you could apply a patch like:

diff --git a/sys/kern/subr_trap.c b/sys/kern/subr_trap.c
index e9a16cd0b36e..54b540c97123 100644
--- a/sys/kern/subr_trap.c
+++ b/sys/kern/subr_trap.c
@@ -138,7 +138,7 @@ userret(struct thread *td, struct trapframe *frame)
         * Software Timer Support for Network Processing"
         * by Mohit Aron and Peter Druschel.
         */
-       tcp_hpts_softclock();
+       /*tcp_hpts_softclock();*/
        /*
         * Let the scheduler adjust our priority etc.
         */


If that fixes it, I suspect we should either make this hook optional for casual users of tcp_hpts(), or add some kind of "last called" timestamp to prevent it being called over and over and over on workloads which are syscall heavy.

Note that for non-casual users of hpts (like Netflix, with hundreds of thousands of TCP connections managed by hpts), this call is a huge win, so I think we'd prefer that it remain in some form.

Drew