Performance of ThreadLocal variable
In 2009, some JVMs implemented ThreadLocal using an unsynchronised HashMap in the Thread.currentThread() object. This made it extremely fast (though not nearly as fast as using a regular field access, of course), as well as ensuring that the ThreadLocal object got tidied up when the Thread died. Updating this answer in 2016, it seems most (all?) newer JVMs use a ThreadLocalMap with linear probing. I am uncertain about the performance of those – but I cannot imagine it is significantly worse than the earlier implementation.
Of course, new Object() is also very fast these days, and the garbage collectors are also very good at reclaiming short-lived objects.
Unless you are certain that object creation is going to be expensive, or you need to persist some state on a thread by thread basis, you are better off going for the simpler allocate when needed solution, and only switching over to a ThreadLocal implementation when a profiler tells you that you need to.
Running unpublished benchmarks, ThreadLocal.get takes around 35 cycle per iteration on my machine. Not a great deal. In Sun's implementation a custom linear probing hash map in Thread maps ThreadLocals to values. Because it is only ever accessed by a single thread, it can be very fast.
Allocation of small objects take a similar number of cycles, although because of cache exhaustion you may get somewhat lower figures in a tight loop.
Construction of MessageDigest is likely to be relatively expensive. It has a fair amount of state and construction goes through the Provider SPI mechanism. You may be able to optimise by, for instance, cloning or providing the Provider.
Just because it may be faster to cache in a ThreadLocal rather than create does not necessarily mean that the system performance will increase. You will have additional overheads related to GC which slows everything down.
Unless your application very heavily uses MessageDigest you might want to consider using a conventional thread-safe cache instead.
Good question, I've been asking myself that recently. To give you definite numbers, the benchmarks below (in Scala, compiled to virtually the same bytecodes as the equivalent Java code):
var cnt: String = ""val tlocal = new java.lang.ThreadLocal[String] { override def initialValue = ""}def loop_heap_write = { var i = 0 val until = totalwork / threadnum while (i < until) { if (cnt ne "") cnt = "!" i += 1 } cnt } def threadlocal = { var i = 0 val until = totalwork / threadnum while (i < until) { if (tlocal.get eq null) i = until + i + 1 i += 1 } if (i > until) println("thread local value was null " + i)}available here, were performed on an AMD 4x 2.8 GHz dual-cores and a quad-core i7 with hyperthreading (2.67 GHz).
These are the numbers:
i7
Specs: Intel i7 2x quad-core @ 2.67 GHzTest: scala.threads.ParallelTests
Test name: loop_heap_read
Thread num.: 1Total tests: 200
Run times: (showing last 5) 9.0069 9.0036 9.0017 9.0084 9.0074 (avg = 9.1034 min = 8.9986 max = 21.0306 )
Thread num.: 2Total tests: 200
Run times: (showing last 5) 4.5563 4.7128 4.5663 4.5617 4.5724 (avg = 4.6337 min = 4.5509 max = 13.9476 )
Thread num.: 4Total tests: 200
Run times: (showing last 5) 2.3946 2.3979 2.3934 2.3937 2.3964 (avg = 2.5113 min = 2.3884 max = 13.5496 )
Thread num.: 8Total tests: 200
Run times: (showing last 5) 2.4479 2.4362 2.4323 2.4472 2.4383 (avg = 2.5562 min = 2.4166 max = 10.3726 )
Test name: threadlocal
Thread num.: 1Total tests: 200
Run times: (showing last 5) 91.1741 90.8978 90.6181 90.6200 90.6113 (avg = 91.0291 min = 90.6000 max = 129.7501 )
Thread num.: 2Total tests: 200
Run times: (showing last 5) 45.3838 45.3858 45.6676 45.3772 45.3839 (avg = 46.0555 min = 45.3726 max = 90.7108 )
Thread num.: 4Total tests: 200
Run times: (showing last 5) 22.8118 22.8135 59.1753 22.8229 22.8172 (avg = 23.9752 min = 22.7951 max = 59.1753 )
Thread num.: 8Total tests: 200
Run times: (showing last 5) 22.2965 22.2415 22.3438 22.3109 22.4460 (avg = 23.2676 min = 22.2346 max = 50.3583 )
AMD
Specs: AMD 8220 4x dual-core @ 2.8 GHzTest: scala.threads.ParallelTests
Test name: loop_heap_read
Total work: 20000000Thread num.: 1Total tests: 200
Run times: (showing last 5) 12.625 12.631 12.634 12.632 12.628 (avg = 12.7333 min = 12.619 max = 26.698 )
Test name: loop_heap_readTotal work: 20000000
Run times: (showing last 5) 6.412 6.424 6.408 6.397 6.43 (avg = 6.5367 min = 6.393 max = 19.716 )
Thread num.: 4Total tests: 200
Run times: (showing last 5) 3.385 4.298 9.7 6.535 3.385 (avg = 5.6079 min = 3.354 max = 21.603 )
Thread num.: 8Total tests: 200
Run times: (showing last 5) 5.389 5.795 10.818 3.823 3.824 (avg = 5.5810 min = 2.405 max = 19.755 )
Test name: threadlocal
Thread num.: 1Total tests: 200
Run times: (showing last 5) 200.217 207.335 200.241 207.342 200.23 (avg = 202.2424 min = 200.184 max = 245.369 )
Thread num.: 2Total tests: 200
Run times: (showing last 5) 100.208 100.199 100.211 103.781 100.215 (avg = 102.2238 min = 100.192 max = 129.505 )
Thread num.: 4Total tests: 200
Run times: (showing last 5) 62.101 67.629 62.087 52.021 55.766 (avg = 65.6361 min = 50.282 max = 167.433 )
Thread num.: 8Total tests: 200
Run times: (showing last 5) 40.672 74.301 34.434 41.549 28.119 (avg = 54.7701 min = 28.119 max = 94.424 )
Summary
A thread local is around 10-20x that of the heap read. It also seems to scale well on this JVM implementation and these architectures with the number of processors.