Re: Request for Testing: TCP RACK

On Sun, 17 Mar 2024 11:40:54 -0400
"Drew Gallatin" <gallatin@freebsd.org> wrote:

> Resending with the patch as an attachment.
> 
> Drew
> 
> On Sun, Mar 17, 2024, at 11:39 AM, Drew Gallatin wrote:
> > 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

Controlled via RW or RWTUN sysctl/tunable?

-- 
Tomoaki AOKI    <junchoon@dec.sakura.ne.jp>