On Thu, Aug 10, 2023 at 2:29 AM Dumitru Ceara <dce...@redhat.com> wrote:
>
> On 8/10/23 08:12, Ales Musil wrote:
> > On Wed, Aug 9, 2023 at 5:13 PM Mark Michelson <mmich...@redhat.com>
wrote:
> >
> >> Hi Ales,
> >>
> >> I have some high-level comments/questions about this patch.
> >>
> >
> > Hi Mark,
> >
>
> Hi Ales, Mark,
>
> > thank you for the review. See my answers inline below.
> >
> >
> >> I have been privy to the conversations that led to this change. My
> >> understanding is that by having ovn-northd wake up immediately, it can
> >> be more CPU-intensive than waiting a bit for changes to accumulate and
> >> handling all of those at once instead. However, nothing in either the
> >> commit message or ovn-nb.xml explains what the purpose of this new
> >> configuration option is. I think you should add a sentence or two to
> >> explain why someone would want to enable this option.
> >>
> >>
> > Yeah that's my bad, I have v2 prepared with some explanation in the
commit
> > message
> > together with results from scale run.
> >
>
> +1 we really need to explain why this change is needed.
>
> >
> >>
> >> Next, the algorithm used here strikes me as odd. We use the previous
run
> >> time of ovn-northd to determine how long to wait before running again.
> >> This delay is capped by the configured backoff time. Let's say that
> >> we've configured the backoff interval to be 200 ms. If ovn-northd has a
> >> super quick run and only takes 10 ms, then we will only delay the next
> >> run by 10 ms. IMO, this seems like it would not mitigate the original
> >> issue by much, since we are only allowing a maximum of 20 ms (10 ms for
> >> the run of ovn-northd + 10 ms delay) of NB changes to accumulate.
> >> Conversely, if northd has a huge recompute and it takes 5000 ms to
> >> complete, then we would delay the next run by 200ms. In this case,
> >> delaying at all seems like it's not necessary since we potentially have
> >> 5000 ms worth of NB DB updates that have not been addressed. I would
> >> have expected the opposite approach to be taken. If someone configures
> >> 200ms as their backoff interval, I would expect us to always allow a
> >> *minimum* of 200ms of NB changes to accumulate before running again. So
> >> for instance, if northd runs quickly and is done in 10 ms, then we
would
> >> wait 200 - 10 = 190 ms before processing changes again. If northd takes
> >> a long time to recompute and takes 5000 ms, then we would not wait at
> >> all before processing changes again. Was the algorithm chosen based on
> >> experimentation? Is it a well-known method I'm just not familiar with?
>
> I think the main assumption (that should probably be made explicit in
> the commit log and/or documentation) is that on average changes happen
> in a uniform way. This might not always be accurate.
>
> However, if we're off with the estimate, in the worst case we'd be
> adding the configured max delay to the latency of processing changes.
> So, as long as the value is not extremely high, the impact is not that
> high either.
>
> Last but not least, as this value would be configured by the CMS, we
> assume the CMS knows what they're doing. :)
>
> >>
> >
> > I'm not sure if the algorithm is well known.
> >
> > The thing is that at scale we almost always cap at the backoff so it has
> > probably
> > the same effect as your suggestion with the difference that we actually
> > delay even
> > after long runs. And that is actually desired, it's true that in the
let's
> > say 500 ms
> > should be enough to accumulate more changes however that can lead to
another
> > 500 ms run and so on. That in the end means that northd will spin at
100%
> > CPU
> > anyway which is what we want to avoid. So from another point of view the
> > accumulation
> > of IDL changes is a secondary effect which is still desired, but not the
> > main purpose.
> >
> > Also delaying by short time if the previous run was short is fine, you
are
> > right that we don't
> > accumulate enough however during short running times there is a high
chance
> > that the
> > northd would get to sleep anyway (We will help it to sleep at least a
bit
> > nevertheless).
> >
> >
> >>
> >> Next, I notice that you've added new poll_timer_wait() calls but
haven't
> >> changed the ovsdb_idl_loop_run() or ovsdb_idl_loop_commit_and_wait()
> >> calls. Is there any danger of ovn-northd getting in a busy loop of
> >> sleeping and waking because of this? I don't think it should, since
> >> presumably ovsdb_idl_loop_run() should clear the conditions waited on
by
> >> ovsdb_idl_loop_commit_and_wait(), but I want to double-check.
> >>
> >
> > AFAIK it shouldn't cause any issues as ovsdb_idl_loop_run() will process
> > anything
> > that it can and wait will be fine. The problem would be if we would
skip the
> > ovsdb_idl_loop_run() for some reason.
> >
> >
> >>
> >> Next, does this have any negative impact on our ability to perform
> >> incremental processing in ovn-northd? My concern is that since we are
> >> still running the ovsdb IDL loop that if multiple NB changes occur
> >> during our delay, then we might have to fall back to a full recompute
> >> instead of being able to incrementally process the changes. Are my
> >> concerns valid?
> >>
> >
> > I suppose that can happen if there are changes that could result in
> > "conflict"
> > and full recompute. During the test we haven't seen anything like that.
> > The odds of that happening are small because as stated previously we are
> > doing
> > basically the same as if the engine was running for a long time always
from
> > the IDL
> > point of view except that we give IDL a chance to process whatever has
> > pilled up
> > within the sleep period.
> >
> >
> >>
> >> Next, has scale testing shown that this change has made a positive
> >> impact? If so, is there any recommendation for how to determine what to
> >> configure the value to?
> >>
> >>
> > It has a huge impact actually the value tested was 200 ms, the
> > recommendation
>
> This was chosen based on the historical data from similar tests which
> showed that the I-P engine was taking ~180-200 ms to run at scale.
>
> > would be < 500 ms. After that point the latency on components creation
> > would be
> > very noticable. I will put the recommendation into the ovn-nb.xml with
the
> > latency
> > comment. Before I'll post v2 (which has the numbers in commit message)
those
> > are the test results:
> >
> > Run without any backoff period:
> > northd aggregate CPU 9810% avg / 12765% max
> > northd was spinning at 100% CPU the entire second half of the test.
> >
> > Run with 200 ms max backoff period:
> > northd aggregate CPU 6066% avg / 7689% max
> > northd was around 60% for the second half of the test
> >
> >
> >>
> >> Finally, is this change expected to be a long-term necessity? This
> >> option seems to be useful for cases where northd recomputes are
> >> required. Performing recomputes less frequently seems like it would
> >> lower the CPU usage of ovn-northd while still processing the same
amount
> >> of changes. However, once northd can handle most changes incrementally,
> >> is there still a benefit to delaying running? If each run of northd
> >> handles all DB changes incrementally, then is there any point in
putting
> >> delays between those incremental runs?
> >>
> >>
> > Ideally we won't need it in the future. However, the assumption for not
> > needing
> > anything like this is that northd will be fast enough to process I-P
> > changes and
> > be able to sleep between the next batch update arrives from CMS. That
> > doesn't
> > seem to happen in very near future, one thing to keep in mind is that
> > testing
> > happened with Numan's I-P for LBs and lflows which make a huge
difference,
> > but
> > still not enough to achieve the mentioned northd state. So from my
> > perspective
> > it will be relevant for a few releases. And as stated above the point
is to
> > prevent
> > northd to spin at 100% CPU all the time.
> >
>
> +1 it's not the prettiest feature (and some might rightfully call it a
> hack) but it seems to me like the cleanest alternative for now, until
> northd processing is fully incremental.
In most cases it may be fine, but it might be a problem for a worst case
scenario:
Assume all the changes coming in NB can be incrementally processed but at
a very very high rate, and ovn-northd keeps processing the changes
incrementally. Since the change rate is so high, ovn-northd barely keeps up
with the changes with 99% CPU load. For example, I-P for each object takes
10ms, and the change rate is 99 objects/sec. According to this algorithm,
ovn-northd will always sleep for the maximum 200ms between each IDL run,
and then ovn-northd would never keep up with the changes any more - the
backlog will become longer and longer because of the wasted idle time.
In practice this means decreased max throughput supported by ovn-northd,
which probably needs to be called out as a tradeoff (in addition to the
latency tradeoff).
Thanks,
Han
>
> Regards,
> Dumitru
>
> >
> >>
> >> On 8/9/23 01:29, Ales Musil wrote:
> >>> Add config option called "northd-backoff-interval-ms" that allows
> >>> to delay northd engine runs capped by the config option.
> >>> When the config option is set to 0 or unspecified, the engine
> >>> will run without any restrictions. If the value >0 we will delay
> >>> northd engine run by the previous run time capped by the
> >>> config option.
> >>>
> >>> Signed-off-by: Ales Musil <amu...@redhat.com>
> >>> ---
> >>> NEWS | 2 ++
> >>> northd/inc-proc-northd.c | 23 ++++++++++++++++++++++-
> >>> northd/inc-proc-northd.h | 3 ++-
> >>> northd/ovn-northd.c | 9 +++++++--
> >>> ovn-nb.xml | 7 +++++++
> >>> 5 files changed, 40 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/NEWS b/NEWS
> >>> index 8275877f9..6109f13a2 100644
> >>> --- a/NEWS
> >>> +++ b/NEWS
> >>> @@ -10,6 +10,8 @@ Post v23.06.0
> >>> - To allow optimizing ovn-controller's monitor conditions for the
> >> regular
> >>> VIF case, ovn-controller now unconditionally monitors all
sub-ports
> >>> (ports with parent_port set).
> >>> + - Add "northd-backoff-interval-ms" config option to delay northd
> >> engine
> >>> + runs capped at the set value.
> >>>
> >>> OVN v23.06.0 - 01 Jun 2023
> >>> --------------------------
> >>> diff --git a/northd/inc-proc-northd.c b/northd/inc-proc-northd.c
> >>> index d328deb22..87db50ad1 100644
> >>> --- a/northd/inc-proc-northd.c
> >>> +++ b/northd/inc-proc-northd.c
> >>> @@ -42,6 +42,8 @@ VLOG_DEFINE_THIS_MODULE(inc_proc_northd);
> >>>
> >>> static unixctl_cb_func chassis_features_list;
> >>>
> >>> +static int64_t next_northd_run_ms = 0;
> >>> +
> >>> #define NB_NODES \
> >>> NB_NODE(nb_global, "nb_global") \
> >>> NB_NODE(logical_switch, "logical_switch") \
> >>> @@ -295,8 +297,10 @@ void inc_proc_northd_init(struct ovsdb_idl_loop
*nb,
> >>> /* Returns true if the incremental processing ended up updating
nodes.
> >> */
> >>> bool inc_proc_northd_run(struct ovsdb_idl_txn *ovnnb_txn,
> >>> struct ovsdb_idl_txn *ovnsb_txn,
> >>> - bool recompute) {
> >>> + bool recompute, uint32_t backoff_ms) {
> >>> ovs_assert(ovnnb_txn && ovnsb_txn);
> >>> +
> >>> + int64_t start = time_msec();
> >>> engine_init_run();
> >>>
> >>> /* Force a full recompute if instructed to, for example, after a
> >> NB/SB
> >>> @@ -330,6 +334,12 @@ bool inc_proc_northd_run(struct ovsdb_idl_txn
> >> *ovnnb_txn,
> >>> } else {
> >>> engine_set_force_recompute(false);
> >>> }
> >>> +
> >>> + int64_t now = time_msec();
> >>> + /* Postpone the next run by length of current run with maximum
> >> capped
> >>> + * by "northd-backoff-interval-ms" interval. */
> >>> + next_northd_run_ms = now + MIN(now - start, backoff_ms);
> >>> +
> >>> return engine_has_updated();
> >>> }
> >>>
> >>> @@ -339,6 +349,17 @@ void inc_proc_northd_cleanup(void)
> >>> engine_set_context(NULL);
> >>> }
> >>>
> >>> +bool
> >>> +inc_proc_northd_can_run(bool recompute)
> >>> +{
> >>> + if (recompute || time_msec() >= next_northd_run_ms) {
> >>> + return true;
> >>> + }
> >>> +
> >>> + poll_timer_wait_until(next_northd_run_ms);
> >>> + return false;
> >>> +}
> >>> +
> >>> static void
> >>> chassis_features_list(struct unixctl_conn *conn, int argc
OVS_UNUSED,
> >>> const char *argv[] OVS_UNUSED, void
*features_)
> >>> diff --git a/northd/inc-proc-northd.h b/northd/inc-proc-northd.h
> >>> index 9b81c7ee0..af418d7d7 100644
> >>> --- a/northd/inc-proc-northd.h
> >>> +++ b/northd/inc-proc-northd.h
> >>> @@ -10,7 +10,8 @@ void inc_proc_northd_init(struct ovsdb_idl_loop *nb,
> >>> struct ovsdb_idl_loop *sb);
> >>> bool inc_proc_northd_run(struct ovsdb_idl_txn *ovnnb_txn,
> >>> struct ovsdb_idl_txn *ovnsb_txn,
> >>> - bool recompute);
> >>> + bool recompute, uint32_t backoff_ms);
> >>> void inc_proc_northd_cleanup(void);
> >>> +bool inc_proc_northd_can_run(bool recompute);
> >>>
> >>> #endif /* INC_PROC_NORTHD */
> >>> diff --git a/northd/ovn-northd.c b/northd/ovn-northd.c
> >>> index 4fa1b039e..3202b50a1 100644
> >>> --- a/northd/ovn-northd.c
> >>> +++ b/northd/ovn-northd.c
> >>> @@ -868,6 +868,7 @@ main(int argc, char *argv[])
> >>> /* Main loop. */
> >>> exiting = false;
> >>>
> >>> + uint32_t northd_backoff_ms = 0;
> >>> bool recompute = false;
> >>> while (!exiting) {
> >>> update_ssl_config();
> >>> @@ -932,10 +933,12 @@ main(int argc, char *argv[])
> >>>
> >>> if (ovsdb_idl_has_lock(ovnsb_idl_loop.idl)) {
> >>> bool activity = false;
> >>> - if (ovnnb_txn && ovnsb_txn) {
> >>> + if (ovnnb_txn && ovnsb_txn &&
> >>> + inc_proc_northd_can_run(recompute)) {
> >>> int64_t loop_start_time = time_wall_msec();
> >>> activity = inc_proc_northd_run(ovnnb_txn,
> >> ovnsb_txn,
> >>> - recompute);
> >>> + recompute,
> >>> +
northd_backoff_ms);
> >>> recompute = false;
> >>> check_and_add_supported_dhcp_opts_to_sb_db(
> >>> ovnsb_txn, ovnsb_idl_loop.idl);
> >>> @@ -1019,6 +1022,8 @@ main(int argc, char *argv[])
> >>> if (nb) {
> >>> interval = smap_get_int(&nb->options,
> >> "northd_probe_interval",
> >>> interval);
> >>> + northd_backoff_ms = smap_get_uint(&nb->options,
> >>> +
> >> "northd-backoff-interval-ms", 0);
> >>> }
> >>> set_idl_probe_interval(ovnnb_idl_loop.idl, ovnnb_db,
interval);
> >>> set_idl_probe_interval(ovnsb_idl_loop.idl, ovnsb_db,
interval);
> >>> diff --git a/ovn-nb.xml b/ovn-nb.xml
> >>> index 4fbf4f7e5..115dfd536 100644
> >>> --- a/ovn-nb.xml
> >>> +++ b/ovn-nb.xml
> >>> @@ -349,6 +349,13 @@
> >>> of HWOL compatibility with GDP.
> >>> </column>
> >>>
> >>> + <column name="options" key="northd-backoff-interval-ms">
> >>> + Maximum interval that the northd incremental engine is
delayed
> >> by
> >>> + in milliseconds. Setting the value to nonzero delays the next
> >> northd
> >>> + engine run by the previous run time, capped by the specified
> >> value.
> >>> + If the value is zero the engine won't be delayed at all.
> >>> + </column>
> >>> +
> >>> <group title="Options for configuring interconnection route
> >> advertisement">
> >>> <p>
> >>> These options control how routes are advertised between
OVN
> >>
> >>
> > Thanks,
> > Ales
> >
>
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