Generating views is extremely slow - makes CouchDB hard to use with non-trivial
number of docs
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Key: COUCHDB-620
URL: https://issues.apache.org/jira/browse/COUCHDB-620
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 them
* 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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