Re: Request for Testing: TCP RACK
- Reply: tuexen_a_freebsd.org: "Re: Request for Testing: TCP RACK"
- In reply to: Drew Gallatin: "Re: Request for Testing: TCP RACK"
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Date: Thu, 28 Mar 2024 14:00:58 UTC
Hello all!
Running rack @b7b78c1c169 "Optimize HPTS..." very happy on my laptop
(amd64)!
Thanks all!
Drew Gallatin <gallatin@freebsd.org> escreveu (quinta, 21/03/2024 à(s)
12:58):
> The entire point is to *NOT* go through the overhead of scheduling
> something asynchronously, but to take advantage of the fact that a
> user/kernel transition is going to trash the cache anyway.
>
> In the common case of a system which has less than the threshold number
> of connections , we access the tcp_hpts_softclock function pointer, make
> one function call, and access hpts_that_need_softclock, and then return.
> So that's 2 variables and a function call.
>
> I think it would be preferable to avoid that call, and to move the
> declaration of tcp_hpts_softclock and hpts_that_need_softclock so that they
> are in the same cacheline. Then we'd be hitting just a single line in the
> common case. (I've made comments on the review to that effect).
>
> Also, I wonder if the threshold could get higher by default, so that hpts
> is never called in this context unless we're to the point where we're
> scheduling thousands of runs of the hpts thread (and taking all those clock
> interrupts).
>
> Drew
>
> On Wed, Mar 20, 2024, at 8:17 PM, Konstantin Belousov wrote:
>
> 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);
>
>
>
>
--
Nuno Teixeira
FreeBSD Committer (ports)