Thanks Chris
[snip, snip, snippety-snip]
CS> What's the database? And the driver?
Oracle 19, oracle.jdbc.OracleDriver - jdbc:oracle:thin.
CS> MySQL Connector/J used to (still does?) read 100% of the results
CS> into the heap before Statement.executeQuery() returns unless you
CS> specifically tell it not to. So if your query returns 1M rows, you
CS> might bust your heap.
CS> It's entirely possible that other drivers do similar things.
The JSON has all the rows, so it appears no pagination is being used on the DB
level.
cb> Multiple TC instances
cb> (3) because multiple copies of the apps don't play nice with each
cb> other. That is, we can't just rename the WAR files and expect the
cb> deployed apps to stay inside that context name (I think).
CS> You might want to look into that, eventually. If they aren't playing
CS> together nicely, they are not "good" servlet citizens. Solving those
CS> issues may improve other things. *shrug*
Yeah, I was working on that previously, but attention spans are short, and I
got pulled off that task onto - SQUIRREL!
cb> StringBuilder - 264MB for the supporting byte array and 264MB for the
cb> returned String, about 790MB total for that piece of the pie.
cb> Contents were simply the JSON query results returned to the client.
cb> No mystery there.
Also, I noticed that the SB internal memory usage is about 2x the size of the
actual contents. Is that because each char is stored as 2 bytes for Unicode?
(Not the char array to string conversion, which is different.)
CS> Yep: runaway string concatenation. This is a devolution of the
CS> "Connector/J reads the whole result set into memory before
CS> returning" thing I mentioned above. Most JSON endpoints
CS> return arbitrarily large JSON responses and most client
CS> applications just go "duh, read the JSON, then process it".
CS> If your JSON is big, well, then you need a lot of memory to
CS> store it all if that' who you do things.
Looking at the contents of the JSON, it's not normalized - a lot of redundant
metadata. Hand-editing the JSON for analysis reduced it from 135 MB to 26 MB.
Maybe the code that generates it can be improved.
CS> If you want to deal with JSON at scale, you need to process
CS> it in a streaming fashion. The only library I know that can do
CS> streaming JSON is Noggit, which was developed for use with
CS> Solr (I think, maybe it came from elsewhere before that).
CS> Anyway, it's ... not for the faint of heart. But if you can figure
CS> out out, you can handle petabytes of JSON with a tiny heap.
I don't think we need to serve up that much data, but I'm guessing we can do
better with what we do serve. Interesting nonetheless.
CS> You might want to throttle/serialize queries you expect to
CS> have big responses so that only e.g. 2 of them can be running
CS> at a time. Maybe all is well when they come one-at-a-time,
CS> but if you try to handle 5 concurrent "big responses" you bust
CS> your heap.
Hmm... I had not thought of throttling that way, restricting the number of
concurrent queries. I was thinking about restricting the number of records
returned. Not sure how to handle lots of users connected but only a few able
to query concurrently. Different DB connection pool with fewer connections for
queries?
cb> (At least StringBuilder is being
cb> used instead of plus-sign String concatenation.)
CS> In Java "..." + "..." uses a StringBuilder
I did not know that. Or I forgot, in which case I can't tell the diff. :-P
CS> In some code, "..." + "..." is just fine
Often it's run-on sentences of plus-sign concatenation with nested quotes,
almost unreadable and even worse for editing. I like to replace with SB for
readability and maintainability.
CS> I hate it when someone replaces it with:
CS> String foo = new StringBuilder("bar").append("baz").toString();
CS> because the compiler does the _exact same thing_ and you've
CS> just made the code more difficult to read.
Ahhh, the classic train wreck. :-)
CS> in a *loop*, then replacing it with a StringBuilder is pretty
CS> important for performance, otherwise the compiler will
CS> do something stupid
I believe the technical term for that is "stoopid". :-) Yeah, I like to be
strategic about SB's and loops.
CS> You might actually have to start reading some code (shiver!).
"You're ... mocking me." :-) Actually, I might be able to pass it off onto
the guy who wrote the library. *phew*
--
Cris Berneburg
CACI Senior Software Engineer
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