Mongo shell query timing out at 90 seconds

Question

I am using mongodb via the mongo shell to query a large collection. For some reason after 90 seconds the mongo shell seems to be stopping my query and nothing is returned.

I have tried the following two commands but neither will return anything. After 90 seconds it just gives me a new line that I can type in another command.

db.cards.find("Field":"Something").maxTimeMS(9999999) db.cards.find("Field":"Something").addOption(DBQuery.Option.tailable)

db.cards.find() return results, but anything with parameters is timing out at exactly 90 seconds and nothing is being returned.

Any help would be greatly appreciated.

Try using `db.cards.find().addOption(DBQuery.Option.noTimeout);`. How many documents do you have in the cards collection? — Rudra, Aug 25 '15 at 04:56

score 2 · Answer 1 · answered Sep 13 '15 at 09:56

Given the level of detail in your question, I am going to focus on 'query a large collection' and guess that your are using the MMAPv1 storage engine, with no index coverage on your query.

Are you disk bound?

Given the above assumptions, you could be cycling data between RAM and disk. Mongo has a default 100MB RAM limit, so if your query has to examine a lot of documents (no index coverage), paging data from disk to RAM could be the culprit. I have heard of mongo shell acting as you describe or locking/terminating when memory constraints are exceeded.

32bit systems can also impose severe memory limits for large collections.

You could look at your OS specific disk activity monitor to get a clue into whether this is your problem.

Just how large is your collection?

Next, how big is your collection? You can show collections and see the physical size of the collection and also db.cards.count() to see your record count. This helps quantify "large collection".

NOTE: you might need the mongo-hacker extensions to see collection disk use in show collections.

Mongo shell investigation

Within the mongo shell, you have a couple more places to look. By default, mongo will log slow queries (> 100ms). After your 90 sec timeout:

db.adminCommand({getLog: "global" })

and look for slow query log entries.

Next look at your winning query plan.

var e = db.cards.explain()
e.find("Field":"Something")

I am guessing you will see

"stage": "COLLSCAN",

Which means you are doing a full collection scan and you need index coverage for your query (good idea for queries and sorts).

Suggestions

You should have at least partial index coverage on any production query. A proper index should solve your problem (assuming you don't have documents > 16MB).

Another approach (that I don't recommend - indexing is better) is to use a cursor instead

var cursor = db.cards.find("Field":"Something")
while (cursor.hasNext()) {
  print(tojson(cursor.next()));
}

Depending on the root cause, this may work for you.

Mongo shell query timing out at 90 seconds

1 Answers1