If you tune for allocation patterns that are in the code, then you are cementing those as continuing in perpetuity. Better to cut the fat first, so that you can tune for the necessary complexity instead of the accidental. That will be self-correcting because any new misuses will be taxed with higher performance regressions.
People started dismissing allocation discipline as a thing from the past because "that thing was solved a lot ago and the compiler now is smart enough".
Well, for string, yes, but not for arbitrary objects.
The JVM does heroics to try and avoid it as much as possible, but when you end up with some primitive boxing in a hotspot the amount of GC pressure that creates can be unreal.
Map<Integer, Integer> intCache = new HashMap<>();
while (loading) {
Integer feild1 = intCache.computeIfAbsent(getField1(), (i)->i);
}
This saves a bunch of memory and ultimately GC pressure as a result.
Valhalla can't come soon enough for us.
To quote Bjarne Stroustrup:
> I don't like garbage. I don't like littering. My ideal is to eliminate the need for a garbage collector by not producing any garbage. That is now possible.
Since there wasn’t a link to the source code in that post, can you help me understand this - for the SLF4J baseline is your logger impl a console appender, a file appender, or a network service like an OTel collector? Does any of that matter for GC context?
These are typically short-lived objects and therefore cheap. Nevertheless, continually creating many such objects increases GC pressure, in particular if the logging happens in code that doesn't otherwise create many objects.
Which then gets discarded because that was a Log.verbose and your minimum log level in production is WARN.
Which is why many libraries have moved towards making your log message returned by a lambda. One constant lambda allocation (so, not a lot, an invokedynamic is absolutely fuck all.) that allows you to straight up skip allocating a full string that most likely is interpolating things and attempting to reach for context present on other threads is strictly better in 99.9% of the cases. The GC pressure is kept minimal and most importantly, constant.
Right now, if I do this
LocalDate a = LocalDate.of(2020, 1, 1);
LocalDate b = LocalDate.of(2020, 1, 1);
In Java, that can be pretty expensive even for an object as light as a LocalDate. By running the cache and doing
var cache = new HashMap<LocalDate, LocalDate>();
LocalDate a = cache.computeIfAbsent(LocalDate.of(2020, 1, 1), (i)->i);
LocalDate b = cache.computeIfAbsent(LocalDate.of(2020, 1, 1), (i)->i);
The technique works best when you have a lot of repeated objects which are immutable. It is also only really needed because Valhalla isn't here. Once "value types" become a thing, then the representation for `LocalDate` inside the JVM can become just the fields and not a reference. The JVM is also free to do the sort of de-duplication optimization all on it's own for larger objects.
This isn't true for the LogEvent or equivalent object, which only gets created after the log level is tested to be applicable by the logger implementation.
For call-site object allocation, you can wrap the logging call into an if statement that checks for the corresponding log level. The lambda allocation isn't constant if it captures anything from the surrounding scope, which will generally be the case for logging calls. (Unless by "constant" you mean that it's a single allocation per execution.)
Consider, for example, if you have a log message like this
logger.info("Hello {}", myOldObject);
That's why it can be faster in some circumstances to write the more wordy
if (logger.isInfoEnabled()) {
logger.info("Hello {}", myOldObject.toString());
}
Why do you think this would happen? There's no mechanism that makes young gen objects that reference old gen objects (or are referenced by old gen objects) get promoted faster. You have to survive a certain number of collections.
I was thinking of "GC Nepotism" [1]. That's the case where an object in old gen pointing to an object in new gen will automatically promote that new gen object into old gen. This can be particularly problematic with graph structures.
[1] https://psy-lob-saw.blogspot.com/2016/03/gc-nepotism-and-lin...
peter_lawrey•5d ago