[ovs-dev] [PATCH ovn] ovn-northd: Fix extremely inefficient usage of lflow hash map.

[Anton Ivanov]

On 23/08/2021 22:36, Ilya Maximets wrote:
> On 8/23/21 10:37 PM, Anton Ivanov wrote:
>> On 23/08/2021 21:26, Ilya Maximets wrote:
>>> On 8/23/21 10:20 PM, Anton Ivanov wrote:
>>>> Should not be the case.
>>>>
>>>> The map is pre-sized for the size from the previous iterations.
>>>>
>>>> Line 12861 in my tree which is probably a few commits out of date:
>>>>
>>>>       fast_hmap_size_for(&lflows, max_seen_lflow_size);
>>>>
>>>> And immediately after building the lflows:
>>>>
>>>>       if (hmap_count(&lflows) > max_seen_lflow_size) {
>>>>           max_seen_lflow_size = hmap_count(&lflows);
>>>>       }
>>>>
>>>> So the map SHOULD be sized correctly - to the most recent seen lflow count.
>>> Please, re-read the commit message.  It's a first run of the loop
>>> and the 'max_seen_lflow_size' is default 128 at this point.
>> Ack,
>>
>> Not using auto-resizing in single threaded mode is a bug. Thanks for fixing it.
>>
>>  From that perspective the patch is a straight +1 from me.
>>
>>  From the perspective of the use case stated in the commit message- I am not sure it addresses it.
>>
>> If northd is in single-threaded mode and is tackling a GIGANTIC> database, it may never complete the first iteration before the
>> expiration of the timers and everyone deciding that northd is AWOL.
> Well, how do you suggest to fix that?  Obviously, we can always create
> a database that northd will never be able to process in a reasonable
> amount of time.  And it doesn't matter if it's single- or multi-threaded.
>
> In this case NbDB is only 9MB in size, which is very reasonable, and
> northd on my laptop takes more than 15 minutes to process it (I killed
> it at this point).  With the patch applied it took only 11 seconds.
> So, for me, this patch pretty much fixes the issue.  11 seconds is not
> that bad, e.g. ovn-k8s configures inactivity probes for clients to 180.
> It would be great to reduce, but we're not there yet.
>
>> In that case, if it is multi-threaded from the start, it should probably
>> grab the sizing from the lflow table hash in south db. That would be a
>> good approximation for the first run.
> This will not work for a case where SbDB is empty for any reason while
> NbDB is not.  And there is still a case where northd initially connects
> to semi-empty databases and after few iterations NbDB receives a big
> transaction and generates a big update for northd.

A partial fix is to resize to optimum size the hash after lflow processing.

If the sizing was correct - 99.9% of the case this will be a noop.

If the sizing was incorrect, it will be resized so that the DP searches 
and all other ops which were recently added for flow reduction will work 
optimally.

This still does not work for lflow compute with dpgroups + parallel upon 
initial connect and without a SB database to use for size guidance. It 
will work for all other cases.

I will send two separate patches to address the cases which can be 
easily addressed and see what can be done with the dp+parallel upon 
initial connect to an empty sb database.

Brgds,

A

>
>> A.
>>
>>>> A.
>>>>
>>>> On 23/08/2021 21:02, Ilya Maximets wrote:
>>>>> 'lflow_map' is never expanded because northd always uses fast
>>>>> insertion.  This leads to the case where we have a hash map
>>>>> with only 128 initial buckets and every ovn_lflow_find() ends
>>>>> up iterating over n_lflows / 128 entries.  It's thousands of
>>>>> logical flows or even more.  For example, it takes forever for
>>>>> ovn-northd to start up with the Northbound Db from the 120 node
>>>>> density-heavy test from ovn-heater, because every lookup is slower
>>>>> than previous one.  I aborted the process after 15 minutes of
>>>>> waiting, because there was no sign that it will converge.  With
>>>>> this change applied the loop completes in only 11 seconds.
>>>>>
>>>>> Hash map will be pre-allocated to the maximum seen number of
>>>>> logical flows on a second iteration, but this doesn't help for
>>>>> the first iteration when northd first time connects to a big
>>>>> Northbound database, which is a common case during failover or
>>>>> cluster upgrade.  And there is an even trickier case where big
>>>>> NbDB transaction that explodes the number of logical flows received
>>>>> on not the first run.
>>>>>
>>>>> We can't expand the hash map in case of parallel build, so this
>>>>> should be fixed separately.
>>>>>
>>>>> CC: Anton Ivanov <anton.ivanov at cambridgegreys.com>
>>>>> Fixes: 74daa0607c7f ("ovn-northd: Introduce parallel lflow build")
>>>>> Signed-off-by: Ilya Maximets <i.maximets at ovn.org>
>>>>> ---
>>>>>     northd/ovn-northd.c | 6 +++++-
>>>>>     1 file changed, 5 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/northd/ovn-northd.c b/northd/ovn-northd.c
>>>>> index 3d8e21a4f..40cf957c0 100644
>>>>> --- a/northd/ovn-northd.c
>>>>> +++ b/northd/ovn-northd.c
>>>>> @@ -4387,7 +4387,11 @@ do_ovn_lflow_add(struct hmap *lflow_map, struct ovn_datapath *od,
>>>>>                        nullable_xstrdup(ctrl_meter),
>>>>>                        ovn_lflow_hint(stage_hint), where);
>>>>>         hmapx_add(&lflow->od_group, od);
>>>>> -    hmap_insert_fast(lflow_map, &lflow->hmap_node, hash);
>>>>> +    if (!use_parallel_build) {
>>>>> +        hmap_insert(lflow_map, &lflow->hmap_node, hash);
>>>>> +    } else {
>>>>> +        hmap_insert_fast(lflow_map, &lflow->hmap_node, hash);
>>>>> +    }
>>>>>     }
>>>>>       /* Adds a row with the specified contents to the Logical_Flow table. */
>

-- 
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661
https://www.cambridgegreys.com/