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From "Stefania (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-11320) Improve backoff policy for cqlsh COPY FROM
Date Thu, 31 Mar 2016 02:31:25 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-11320?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15219221#comment-15219221
] 

Stefania commented on CASSANDRA-11320:
--------------------------------------

Thank you for your review.   

bq. The max backoff of 2^32 seconds is way too high. I feel like we should start lower (maybe
10ms?) and set maxbackoffattempts to 12, for a max sleep of ~40 seconds.
    
I've changed the default value of maxbackoffattempts to 12 and multiplied the back-off delay
by 0.01, which should give sleep periods from 10 ms to 40 seconds max.

bq. I'm also not sure if we should expose the three new config options to users. They're advanced
enough that I don't think anybody will realistically be able to tune them well. As long as
we pick decent defaults, I think they're okay being internal only. (Plus, in a worst-case
scenario, you can directly edit the .py files.)
    
I've removed the config options from the documentation and command line completion. I agree
that most users would not really know how to tune them but we need a way to change them for
testing. BTW, the test code is [here|https://github.com/stef1927/cassandra-dtest/commits/11320].
I believe they should be fine as hidden options?

bq. In feed() (inside init_feeding_thread()), should we only be debug printing the exception?
It seems like we should at least be using printmsg(), but I could be missing a good reason
to keep it at the debug level. If there is a good reason, add a code comment about it.

It should have been {{printmsg}}, thanks.
    
bq. I think filter_replica() can be renamed to replica_is_not_overloaded() for more clarity.
    
OK.

bq. ConnectionWrapper.connections can be declared inside of the class as a class-level attribute.

OK.    

bq. Should we make maxinserterrors default to high non-negative value, something like 1k?
Right now, if your cluster goes down, it can take a long time for COPY to error out the entire
CSV file.

That's a good point, thank you.


{quote}
There's also a pretty minor issue: with --debug and a node that's slow to respond, I sometimes
see a few {{ImportError}}s like this:

{code}
Replicas too busy, given up
Traceback (most recent call last):
  File "/home/thobbs/cassandra/bin/cqlsh.py", line 2618, in main
    from tzlocal import get_localzone
ImportError: No module named tzlocal
{code}
{quote}

I've fixed this as follows:

{code}
if self.debug and sys.exc_info()[1] == err:
    traceback.print_exc()
{code}

Is there a better way?

--

I've restarted CI on the 3.0 branch. Although this patch is labeled as an improvement, it
is a workaround for a potential problem in the driver, see [PYTHON-514|https://datastax-oss.atlassian.net/browse/PYTHON-514].
If the Cassandra host is very slow, we can reach 32k pending requests and at this point COPY
FROM would not recover from {{NoHostAvailable}} exceptions. This is why I've targeted 3.0
rather than trunk. We could back-port it to 2.2 as well. Let me know what your thoughts are
and, once the code review is completed, I will take care of merging to other branches and
running CI.

> Improve backoff policy for cqlsh COPY FROM
> ------------------------------------------
>
>                 Key: CASSANDRA-11320
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11320
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Tools
>            Reporter: Stefania
>            Assignee: Stefania
>              Labels: doc-impacting
>             Fix For: 3.0.x, 3.x
>
>
> Currently we have an exponential back-off policy in COPY FROM that kicks in when timeouts
are received. However there are two limitations:
> * it does not cover new requests and therefore we may not back-off sufficiently to give
time to an overloaded server to recover
> * the pause is performed in the receiving thread and therefore we may not process server
messages quickly enough
> There is a static throttling mechanism in rows per second from feeder to worker processes
(the INGESTRATE) but the feeder has no idea of the load of each worker process. However it's
easy to keep track of how many chunks a worker process has yet to read by introducing a bounded
semaphore.
> The idea is to move the back-off pauses to the worker processes main thread so as to
include all messages, new and retries, not just the retries that timed out. The worker process
will not read new chunks during the back-off pauses, and the feeder process can then look
at the number of pending chunks before sending new chunks to a worker process.
> [~aholmber], [~aweisberg] what do you think?  



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