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From Joan Touzet <woh...@apache.org>
Subject Re: [DISCUSS] Improve load shedding by enforcing timeouts throughout stack
Date Fri, 26 Apr 2019 20:19:44 GMT
Hi Adam,

I'll bring up a concern from a recent client with whom I engaged.

They're on 1.x. On 1.x they have been doing 50k bulk update operations 
in a single request. 1.x doesn't time out. The updates are such that 
they guarantee that none will result in a conflict or be rejected, so 
all 50k are accepted. They do this so it appears atomic to the next 
reader - a read from another client can't occur in the middle of the big 
update, because we have a single couch_file in 1.x.

Obviously, in 2.x this doesn't work on two levels. First, there's 
multiple readers and writers across a cluster, so the big bulk operation 
doesn't act as a blocker until it's finished for any interposed reads. 
Second, you can't reliably finish 50k updates in a single batch in a 
cluster anyway, because you'll probably hit the fabric timeout, if not 
other cluster timeouts.

As a general rule of thumb, I advise people to keep bulk document 
updates to no more than batches of 1k at a time, with the understanding 
that in 2.x these are not treated as an atomic transaction (and they 
weren't strictly that way in 1.x, either, but never mind that...)

If we decide as a project that all operations must take less than 5 
seconds, we're probably going to have to reduce the bulk update batch 
size even further. I'm betting 100 would be the upper bound on bulk updates.

Is this going to impose a significant performance penalty on bulk ops?

-Joan

On 2019-04-26 3:30 p.m., Adam Kocoloski wrote:
> Hi all,
> 
> The point I’m on is that we should take advantage of this extra bit of information
that we acquire out-of-band (e.g. we just decide as a project that all operations take less
than 5 seconds) and come up with smarter / cheaper / faster ways of doing load shedding based
on that information.
> 
> For example, yes it could be interesting to use is_process_alive/1 to see if a client
is still hanging around, and have the gen_server discard the work otherwise. It might also
be too expensive to matter; I’m not sure anyone here has a good a priori sense of the cost
of that call. But I’d certainly wager it’s more expensive than calling timer:now_diff/2
in the server and discarding any requests that were submitted more than 5 seconds ago.
> 
> Most of our timeout / cleanup solutions to date have been focused top-down, without making
any assumptions about the behavior of the workers or servers underneath. I think we should
try to approach this problem bottoms-up, forcing every call to complete within 5 seconds and
handling timeouts correctly as they bubble up.
> 
> Adam
> 
>> On Apr 23, 2019, at 2:48 PM, Nick Vatamaniuc <vatamane@gmail.com> wrote:
>>
>> We don't spawn (/link) or monitor remote processes, just monitor the local
>> coordinator process. That should cheaper performance-wise. It's also for
>> relatively long running streaming fabric requests (changes, all_docs). But
>> you're right, perhaps doing these for shorter requests (doc updates, doc
>> GETs) might become noticeable. Perhaps a pool of reusable monitoring
>> processes work there...
>>
>> For couch_server timeouts. I wonder if we can do a simpler thing and
>> inspect the `From` part of each call and if the Pid is not alive drop the
>> requestor at least avoid doing any expensive processing. For casts it might
>> involve sending a sender Pid in the message. That doesn't address timeouts,
>> just the case where the coordinating process went away while the message
>> was stuck in the long message queue.
>>
>> On Mon, Apr 22, 2019 at 4:32 PM Robert Newson <rnewson@apache.org> wrote:
>>
>>> My memory is fuzzy, but those items sound a lot like what happens with
>>> rex, that motivated us (i.e, Adam) to build rexi, which deliberately does
>>> less than the stock approach.
>>>
>>> --
>>>   Robert Samuel Newson
>>>   rnewson@apache.org
>>>
>>> On Mon, 22 Apr 2019, at 18:33, Nick Vatamaniuc wrote:
>>>> Hi everyone,
>>>>
>>>> We partially implement the first part (cleaning rexi workers) for all
>>>> the
>>>> fabric streaming requests. Which should be all_docs, changes, view map,
>>>> view reduce:
>>>>
>>> https://github.com/apache/couchdb/commit/632f303a47bd89a97c831fd0532cb7541b80355d
>>>>
>>>> The pattern there is the following:
>>>>
>>>> - With every request spawn a monitoring process that is in charge of
>>>> keeping track of all the workers as they are spawned.
>>>> - If regular cleanup takes place, then this monitoring process is
>>> killed,
>>>> to avoid sending double the number of kill messages to workers.
>>>> - If the coordinating process doesn't run cleanup and just dies, the
>>>> monitoring process will performs cleanup on its behalf.
>>>>
>>>> Cheers,
>>>> -Nick
>>>>
>>>>
>>>>
>>>> On Thu, Apr 18, 2019 at 5:16 PM Robert Samuel Newson <rnewson@apache.org
>>>>
>>>> wrote:
>>>>
>>>>> My view is a) the server was unavailable for this request due to all
>>> the
>>>>> other requests it’s currently dealing with b) the connection was not
>>> idle,
>>>>> the client is not at fault.
>>>>>
>>>>> B.
>>>>>
>>>>>> On 18 Apr 2019, at 22:03, Done Collectively <sansato@inator.biz>
>>> wrote:
>>>>>>
>>>>>> Any reason 408 would be undesirable?
>>>>>>
>>>>>> https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/408
>>>>>>
>>>>>>
>>>>>> On Thu, Apr 18, 2019 at 10:37 AM Robert Newson <rnewson@apache.org>
>>>>> wrote:
>>>>>>
>>>>>>> 503 imo.
>>>>>>>
>>>>>>> --
>>>>>>> Robert Samuel Newson
>>>>>>> rnewson@apache.org
>>>>>>>
>>>>>>> On Thu, 18 Apr 2019, at 18:24, Adam Kocoloski wrote:
>>>>>>>> Yes, we should. Currently it’s a 500, maybe there’s something
more
>>>>>>> appropriate:
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>
>>> https://github.com/apache/couchdb/blob/8ef42f7241f8788afc1b6e7255ce78ce5d5ea5c3/src/chttpd/src/chttpd.erl#L947-L949
>>>>>>>>
>>>>>>>> Adam
>>>>>>>>
>>>>>>>>> On Apr 18, 2019, at 12:50 PM, Joan Touzet <wohali@apache.org>
>>> wrote:
>>>>>>>>>
>>>>>>>>> What happens when it turns out the client *hasn't* timed
out and
>>> we
>>>>>>>>> just...hang up on them? Should we consider at least trying
to send
>>>>> back
>>>>>>>>> some sort of HTTP status code?
>>>>>>>>>
>>>>>>>>> -Joan
>>>>>>>>>
>>>>>>>>> On 2019-04-18 10:58, Garren Smith wrote:
>>>>>>>>>> I'm +1 on this. With partition queries, we added
a few more
>>> timeouts
>>>>>>> that
>>>>>>>>>> can be enabled which Cloudant enable. So having the
ability to
>>> shed
>>>>>>> old
>>>>>>>>>> requests when these timeouts get hit would be great.
>>>>>>>>>>
>>>>>>>>>> Cheers
>>>>>>>>>> Garren
>>>>>>>>>>
>>>>>>>>>> On Tue, Apr 16, 2019 at 2:41 AM Adam Kocoloski <
>>> kocolosk@apache.org>
>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>> Hi all,
>>>>>>>>>>>
>>>>>>>>>>> For once, I’m coming to you with a topic that
is not strictly
>>> about
>>>>>>>>>>> FoundationDB :)
>>>>>>>>>>>
>>>>>>>>>>> CouchDB offers a few config settings (some of
them
>>> undocumented) to
>>>>>>> put a
>>>>>>>>>>> limit on how long the server is allowed to take
to generate a
>>>>>>> response. The
>>>>>>>>>>> trouble with many of these timeouts is that,
when they fire,
>>> they do
>>>>>>> not
>>>>>>>>>>> actually clean up all of the work that they initiated.
A couple
>>> of
>>>>>>> examples:
>>>>>>>>>>>
>>>>>>>>>>> - Each HTTP response coordinated by the “fabric”
application
>>> spawns
>>>>>>>>>>> several ephemeral processes via “rexi" on different
nodes in the
>>>>>>> cluster to
>>>>>>>>>>> retrieve data and send it back to the process
coordinating the
>>>>>>> response. If
>>>>>>>>>>> the request timeout fires, the coordinating process
will be
>>> killed
>>>>>>> off, but
>>>>>>>>>>> the ephemeral workers might not be. In a healthy
cluster they’ll
>>>>>>> exit on
>>>>>>>>>>> their own when they finish their jobs, but there
are conditions
>>>>>>> under which
>>>>>>>>>>> they can sit around for extended periods of time
waiting for an
>>>>>>> overloaded
>>>>>>>>>>> gen_server (e.g. couch_server) to respond.
>>>>>>>>>>>
>>>>>>>>>>> - Those named gen_servers (like couch_server)
responsible for
>>>>>>> serializing
>>>>>>>>>>> access to important data structures will dutifully
process
>>> messages
>>>>>>>>>>> received from old requests without any regard
for (of even
>>> knowledge
>>>>>>> of)
>>>>>>>>>>> the fact that the client that sent the message
timed out long
>>> ago.
>>>>>>> This can
>>>>>>>>>>> lead to a sort of death spiral in which the gen_server
is
>>> ultimately
>>>>>>>>>>> spending ~all of its time serving dead clients
and every client
>>> is
>>>>>>> timing
>>>>>>>>>>> out.
>>>>>>>>>>>
>>>>>>>>>>> I’d like to see us introduce a documented maximum
request
>>> duration
>>>>>>> for all
>>>>>>>>>>> requests except the _changes feed, and then use
that
>>> information to
>>>>>>> aid in
>>>>>>>>>>> load shedding throughout the stack. We can audit
the codebase
>>> for
>>>>>>>>>>> gen_server calls with long timeouts (I know of
a few on the
>>> critical
>>>>>>> path
>>>>>>>>>>> that set their timeouts to `infinity`) and we
can design servers
>>>>> that
>>>>>>>>>>> efficiently drop old requests, knowing that the
client who made
>>> the
>>>>>>> request
>>>>>>>>>>> must have timed out. A couple of topics for discussion:
>>>>>>>>>>>
>>>>>>>>>>> - the “gen_server that sheds old requests”
is a very generic
>>>>>>> pattern, one
>>>>>>>>>>> that seems like it could be well-suited to its
own behaviour. A
>>>>>>> cursory
>>>>>>>>>>> search of the internet didn’t turn up any prior
art here, which
>>>>>>> surprises
>>>>>>>>>>> me a bit. I’m wondering if this is worth bringing
up with the
>>>>> broader
>>>>>>>>>>> Erlang community.
>>>>>>>>>>>
>>>>>>>>>>> - setting and enforcing timeouts is a healthy
pattern for
>>> read-only
>>>>>>>>>>> requests as it gives a lot more feedback to clients
about the
>>> health
>>>>>>> of the
>>>>>>>>>>> server. When it comes to updates things are a
little bit more
>>> muddy,
>>>>>>> just
>>>>>>>>>>> because there remains a chance that an update
can be committed,
>>> but
>>>>>>> the
>>>>>>>>>>> caller times out before learning of the successful
commit. We
>>> should
>>>>>>> try to
>>>>>>>>>>> minimize the likelihood of that occurring.
>>>>>>>>>>>
>>>>>>>>>>> Cheers, Adam
>>>>>>>>>>>
>>>>>>>>>>> P.S. I did say that this wasn’t _strictly_
about FoundationDB,
>>> but
>>>>> of
>>>>>>>>>>> course FDB has a hard 5 second limit on all transactions,
so it
>>> is a
>>>>>>> bit of
>>>>>>>>>>> a forcing function :).Even putting FoundationDB
aside, I would
>>> still
>>>>>>> argue
>>>>>>>>>>> to pursue this path based on our Ops experience
with the current
>>>>>>> codebase.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>
>>>>>
>>>>
>>>
> 

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