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From "Roger Binns (JIRA)" <>
Subject [jira] Commented: (COUCHDB-620) Generating views is extremely slow - makes CouchDB hard to use with non-trivial number of docs
Date Mon, 11 Jan 2010 16:34:54 GMT


Roger Binns commented on COUCHDB-620:

I currently have a 4 core machine (and 2 cores until a few weeks ago).  Less than one core
in total is used between couchjs and beam.smp (erlang).  Even if the changes in 495 make it
go twice as fast, the performance is still abysmal - I'd still be measuring view generation
time in hours and I'd still have mostly idle CPU and I/O.  Even having there be one less couchjs
than cores would be fine if you are that worried.  (The ideal number would depend on division
 of labour during the view generation between erlang and couchjs.)

In summary please saturate something - CPU, RAM or I/O when doing view generation.  Anything
less than that means that the full resources of the machine are not being used and there is
extra unnecessary wait for the view generation completion.  I'd really like to stop measuring
the time taken in hours.

> Generating views is extremely slow - makes CouchDB hard to use with non-trivial number
of docs
> ----------------------------------------------------------------------------------------------
>                 Key: COUCHDB-620
>                 URL:
>             Project: CouchDB
>          Issue Type: Improvement
>          Components: Infrastructure
>    Affects Versions: 0.10
>         Environment: Ubuntu 9.10 64 bit, CouchDB 0.10
>            Reporter: Roger Binns
> Generating views is extremely slow.  For example adding 10 million documents takes less
than 10 minutes but generating some simple views on the same docs takes over 4 hours.
> Using top you can see that CouchDB (erlang) and couchjs between them cannot even saturate
a single CPU let alone the I/O system.  Under ideal conditions performance should be limited
by cpu, disk or memory.  This implies that the processes are doing simple things in lockstep
accumulating latencies in each process as well as the communication between them which when
multiplied by the number of documents can amount to a lot.
> Some suggestions:
> * Run as many couchjs instances as there are processor cores and scatter work amongst
> * Have some sort of pipelining in the erlang so that the moment the first byte of response
is received from couchjs the data is sent for the next request (the JSON conversion, HTTP
headers etc should all have been assembled already) to reduce latencies.  Do whatever is most
similar in couchjs (eg use separate threads to read requests, process them and write responses).
> * Use the equivalent of HTTP pipelining when talking to couchjs so that it always has
a doc ready to work on rather than having to transmit an entire response and then wait for
erlang to think and provide an entire new request
> A simple test of success is to have a database with a million or so documents with a
trivial view and have view creation max out the CPU,. memory or disk.
> Some things in CouchDB make this a particularly nasty problem.  View data is not replicated
so replicating documents can lead the view data by a large margin on the recipient database.
 This can lead to inconsistencies.  You also can't expect users to then wait minutes (or hours)
for a request to complete because the view generation got that far behind.  (My own plans
now are to not use replication and instead create the database file on another couchdb instance
and then rsync the binary database file over instead!)
> Although stale=ok is available, you still have no idea if the response will be quick
or take however long view generation does.  (Sure I could add some sort of timeout and complicate
the code but then what value do I pick?  If I have a user waiting I want an answer ASAP or
I have to give them some horrible error message.  Taking a long wait and then giving a timeout
is even worse!)

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