I'm trying to test the performance of MongoDB before actually putting it to use. I'm trying to see how many documents can I update per second. I'm using C# (Mono + Ubuntu) MongoDB Driver v1.9 and MongoDB v2.4.6.
I believe one of the most effective MongoDB parameters on write performance is Write Concern. As it is stated in documentation, the most relaxed value for write concern would be -1, then 0 and finally 1 is the slowest one.
After searching I found that I can set write concerns in C# embedded in connection string like this:
var client = new MongoClient("mongodb://localhost/?w=-1");
Here are the results of me playing around with different values for w:
- Fastest results are achieved when I set
wto1! - Setting
w=0is slower thanw=128 times! w=-1will lead to an exception thrown with error messageW value must be greater than or equal to zero!
Does anyone have any explanations on these results? Am I doing anything wrong?
[UPDATE]
I think it is necessary to set the test procedure maybe there's something hidden within it. So here it goes:
I've got a database with 100M documents in a single collection. Each document is created like this using mongo shell:
{ "counter" : 0, "last_update" : new Date() }
Here's the output of db.collection.stats();:
{
"ns" : "test.collection",
"count" : 100000100,
"size" : 6400006560,
"avgObjSize" : 64.0000015999984,
"storageSize" : 8683839472,
"numExtents" : 27,
"nindexes" : 2,
"lastExtentSize" : 2146426864,
"paddingFactor" : 1,
"systemFlags" : 1,
"userFlags" : 0,
"totalIndexSize" : 5769582448,
"indexSizes" : {
"_id_" : 3251652432,
"last_update_1" : 2517930016
},
"ok" : 1
}
Using Mono 3.2.1 in Ubuntu 12.04 I've written a C# project which connects to MongoDB and tries to update the documents like this:
FindAndModifyArgs args = new FindAndModifyArgs();
args.SortBy = SortBy.Ascending("last_update");
args.Update = Update<Entity>.Set(e => e.last_update, DateTime.Now);
args.Fields = Fields.Include(new string[] { "counter", "_id" });
var m = collection.FindAndModify(args);
Entity ent = m.GetModifiedDocumentAs<Entity>();
var query = Query<Entity>.EQ(e => e.Id, ent.Id);
var update = Update<Entity>.Set(e => e.counter, ent.counter+1);
collection.Update(query, update);
To summarize what this piece of code does; it selects the oldest last_update and while it sets the last_update to current date, it also increments its counter (update happens in two steps).
I ran this code 10k for each of four different types of Write Concerns, w=-1, w=0, w=1 and w=1&j=true. While w=-1 throws an exception and gives out no results, here are the results for the rest of them:
Since the figure is a little hard to read, here're the same results in numbers:
w=-1 w=0 w=1 w=1&j=true
Average N/A 244.0948611492 7064.5143923477 238.1846428156
STDEV N/A 1.7787457992 511.892765742 21.0230097306
And the question is: Does anyone have any explanations why w=0 is much slower than w=1 and why w=-1 is not supported?
[UPDATE]
I've also tested RequestStart in my code like this:
using (server.RequestStart(database)) {
FindAndModifyArgs args = new FindAndModifyArgs();
args.SortBy = SortBy.Ascending("last_update");
args.Update = Update<Entity>.Set(e => e.last_update, DateTime.Now);
args.Fields = Fields.Include(new string[] { "counter", "_id" });
var m = collection.FindAndModify(args);
Entity ent = m.GetModifiedDocumentAs<Entity>();
var query = Query<Entity>.EQ(e => e.Id, ent.Id);
var update = Update<Entity>.Set(e => e.counter, ent.counter+1);
collection.Update(query, update);
}
It had no significant effect on any of the results, what so ever.