Re: Request for Testing: TCP RACK
- Reply: Drew Gallatin: "Re: Request for Testing: TCP RACK"
- Go to: [ bottom of page ] [ top of archives ] [ this month ]
Date: Thu, 21 Mar 2024 00:17:37 UTC
On Tue, Mar 19, 2024 at 06:19:52AM -0400, rrs wrote:
> Ok I have created
>
> https://reviews.freebsd.org/D44420
>
>
> To address the issue. I also attach a short version of the patch that Nuno
> can try and validate
>
> it works. Drew you may want to try this and validate the optimization does
> kick in since I can
>
> only now test that it does not on my local box :)
The patch still causes access to all cpu's cachelines on each userret.
It would be much better to inc/check the threshold and only schedule the
call when exceeded. Then the call can occur in some dedicated context,
like per-CPU thread, instead of userret.
>
>
> R
>
>
>
> On 3/18/24 3:42 PM, Drew Gallatin wrote:
> > No. The goal is to run on every return to userspace for every thread.
> >
> > Drew
> >
> > On Mon, Mar 18, 2024, at 3:41 PM, Konstantin Belousov wrote:
> > > On Mon, Mar 18, 2024 at 03:13:11PM -0400, Drew Gallatin wrote:
> > > > I got the idea from
> > > > https://people.mpi-sws.org/~druschel/publications/soft-timers-tocs.pdf
> > > > The gist is that the TCP pacing stuff needs to run frequently, and
> > > > rather than run it out of a clock interrupt, its more efficient to run
> > > > it out of a system call context at just the point where we return to
> > > > userspace and the cache is trashed anyway. The current implementation
> > > > is fine for our workload, but probably not idea for a generic system.
> > > > Especially one where something is banging on system calls.
> > > >
> > > > Ast's could be the right tool for this, but I'm super unfamiliar with
> > > > them, and I can't find any docs on them.
> > > >
> > > > Would ast_register(0, ASTR_UNCOND, 0, func) be roughly equivalent to
> > > > what's happening here?
> > > This call would need some AST number added, and then it registers the
> > > ast to run on next return to userspace, for the current thread.
> > >
> > > Is it enough?
> > > >
> > > > Drew
> > >
> > > >
> > > > On Mon, Mar 18, 2024, at 2:33 PM, Konstantin Belousov wrote:
> > > > > On Mon, Mar 18, 2024 at 07:26:10AM -0500, Mike Karels wrote:
> > > > > > On 18 Mar 2024, at 7:04, tuexen@freebsd.org wrote:
> > > > > >
> > > > > > >> On 18. Mar 2024, at 12:42, Nuno Teixeira
> > > <eduardo@freebsd.org> wrote:
> > > > > > >>
> > > > > > >> Hello all!
> > > > > > >>
> > > > > > >> It works just fine!
> > > > > > >> System performance is OK.
> > > > > > >> Using patch on main-n268841-b0aaf8beb126(-dirty).
> > > > > > >>
> > > > > > >> ---
> > > > > > >> net.inet.tcp.functions_available:
> > > > > > >> Stack D
> > > Alias PCB count
> > > > > > >> freebsd freebsd 0
> > > > > > >> rack *
> > > rack 38
> > > > > > >> ---
> > > > > > >>
> > > > > > >> It would be so nice that we can have a sysctl tunnable for
> > > this patch
> > > > > > >> so we could do more tests without recompiling kernel.
> > > > > > > Thanks for testing!
> > > > > > >
> > > > > > > @gallatin: can you come up with a patch that is acceptable
> > > for Netflix
> > > > > > > and allows to mitigate the performance regression.
> > > > > >
> > > > > > Ideally, tcphpts could enable this automatically when it
> > > starts to be
> > > > > > used (enough?), but a sysctl could select auto/on/off.
> > > > > There is already a well-known mechanism to request execution of the
> > > > > specific function on return to userspace, namely AST. The difference
> > > > > with the current hack is that the execution is requested for one
> > > callback
> > > > > in the context of the specific thread.
> > > > >
> > > > > Still, it might be worth a try to use it; what is the reason to
> > > hit a thread
> > > > > that does not do networking, with TCP processing?
> > > > >
> > > > > >
> > > > > > Mike
> > > > > >
> > > > > > > Best regards
> > > > > > > Michael
> > > > > > >>
> > > > > > >> Thanks all!
> > > > > > >> Really happy here :)
> > > > > > >>
> > > > > > >> Cheers,
> > > > > > >>
> > > > > > >> Nuno Teixeira <eduardo@freebsd.org> escreveu (domingo,
> > > 17/03/2024 à(s) 20:26):
> > > > > > >>>
> > > > > > >>> Hello,
> > > > > > >>>
> > > > > > >>>> 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:
> > > > > > >>>
> > > > > > >>> It's affecting overall system performance, bonnie was just
> > > a way to
> > > > > > >>> get some numbers to compare.
> > > > > > >>>
> > > > > > >>> Tomorrow I will test patch.
> > > > > > >>>
> > > > > > >>> Thanks!
> > > > > > >>>
> > > > > > >>> --
> > > > > > >>> Nuno Teixeira
> > > > > > >>> FreeBSD Committer (ports)
> > > > > > >>
> > > > > > >>
> > > > > > >>
> > > > > > >> --
> > > > > > >> Nuno Teixeira
> > > > > > >> FreeBSD Committer (ports)
> > > > > >
> > > > >
> > >
> >
> diff --git a/sys/netinet/tcp_hpts.c b/sys/netinet/tcp_hpts.c
> index 8c4d2d41a3eb..eadbee19f69c 100644
> --- a/sys/netinet/tcp_hpts.c
> +++ b/sys/netinet/tcp_hpts.c
> @@ -216,6 +216,7 @@ struct tcp_hpts_entry {
> void *ie_cookie;
> uint16_t p_num; /* The hpts number one per cpu */
> uint16_t p_cpu; /* The hpts CPU */
> + uint8_t hit_callout_thresh;
> /* There is extra space in here */
> /* Cache line 0x100 */
> struct callout co __aligned(CACHE_LINE_SIZE);
> @@ -269,6 +270,11 @@ static struct hpts_domain_info {
> int cpu[MAXCPU];
> } hpts_domains[MAXMEMDOM];
>
> +counter_u64_t hpts_that_need_softclock;
> +SYSCTL_COUNTER_U64(_net_inet_tcp_hpts_stats, OID_AUTO, needsoftclock, CTLFLAG_RD,
> + &hpts_that_need_softclock,
> + "Number of hpts threads that need softclock");
> +
> counter_u64_t hpts_hopelessly_behind;
>
> SYSCTL_COUNTER_U64(_net_inet_tcp_hpts_stats, OID_AUTO, hopeless, CTLFLAG_RD,
> @@ -334,7 +340,7 @@ SYSCTL_INT(_net_inet_tcp_hpts, OID_AUTO, precision, CTLFLAG_RW,
> &tcp_hpts_precision, 120,
> "Value for PRE() precision of callout");
> SYSCTL_INT(_net_inet_tcp_hpts, OID_AUTO, cnt_thresh, CTLFLAG_RW,
> - &conn_cnt_thresh, 0,
> + &conn_cnt_thresh, DEFAULT_CONNECTION_THESHOLD,
> "How many connections (below) make us use the callout based mechanism");
> SYSCTL_INT(_net_inet_tcp_hpts, OID_AUTO, logging, CTLFLAG_RW,
> &hpts_does_tp_logging, 0,
> @@ -1548,6 +1554,9 @@ __tcp_run_hpts(void)
> struct tcp_hpts_entry *hpts;
> int ticks_ran;
>
> + if (counter_u64_fetch(hpts_that_need_softclock) == 0)
> + return;
> +
> hpts = tcp_choose_hpts_to_run();
>
> if (hpts->p_hpts_active) {
> @@ -1683,6 +1692,13 @@ tcp_hpts_thread(void *ctx)
> ticks_ran = tcp_hptsi(hpts, 1);
> tv.tv_sec = 0;
> tv.tv_usec = hpts->p_hpts_sleep_time * HPTS_TICKS_PER_SLOT;
> + if ((hpts->p_on_queue_cnt > conn_cnt_thresh) && (hpts->hit_callout_thresh == 0)) {
> + hpts->hit_callout_thresh = 1;
> + counter_u64_add(hpts_that_need_softclock, 1);
> + } else if ((hpts->p_on_queue_cnt <= conn_cnt_thresh) && (hpts->hit_callout_thresh == 1)) {
> + hpts->hit_callout_thresh = 0;
> + counter_u64_add(hpts_that_need_softclock, -1);
> + }
> if (hpts->p_on_queue_cnt >= conn_cnt_thresh) {
> if(hpts->p_direct_wake == 0) {
> /*
> @@ -1818,6 +1834,7 @@ tcp_hpts_mod_load(void)
> cpu_top = NULL;
> #endif
> tcp_pace.rp_num_hptss = ncpus;
> + hpts_that_need_softclock = counter_u64_alloc(M_WAITOK);
> hpts_hopelessly_behind = counter_u64_alloc(M_WAITOK);
> hpts_loops = counter_u64_alloc(M_WAITOK);
> back_tosleep = counter_u64_alloc(M_WAITOK);
> @@ -2042,6 +2059,7 @@ tcp_hpts_mod_unload(void)
> free(tcp_pace.grps, M_TCPHPTS);
> #endif
>
> + counter_u64_free(hpts_that_need_softclock);
> counter_u64_free(hpts_hopelessly_behind);
> counter_u64_free(hpts_loops);
> counter_u64_free(back_tosleep);