I/O and non-blocking I/O selection depends from your server activity profile. E.g. if you use long-living connections and thousands of clients I/O may become too expensive because of system resources exhaustion. However, direct I/O that doesn't crowd out CPU cache is faster than non-blocking I/O. There is a good article about that - Writing Java Multithreaded Servers - whats old is new.
About context switch cost - it's rather chip operation. Consider the simple test below:
package com;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.Set;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
public class AAA {
private static final long DURATION = TimeUnit.NANOSECONDS.convert(30, TimeUnit.SECONDS);
private static final int THREADS_NUMBER = 2;
private static final ThreadLocal<AtomicLong> COUNTER = new ThreadLocal<AtomicLong>() {
@Override
protected AtomicLong initialValue() {
return new AtomicLong();
}
};
private static final ThreadLocal<AtomicLong> DUMMY_DATA = new ThreadLocal<AtomicLong>() {
@Override
protected AtomicLong initialValue() {
return new AtomicLong();
}
};
private static final AtomicLong DUMMY_COUNTER = new AtomicLong();
private static final AtomicLong END_TIME = new AtomicLong(System.nanoTime() + DURATION);
private static final List<ThreadLocal<CharSequence>> DUMMY_SOURCE = new ArrayList<ThreadLocal<CharSequence>>();
static {
for (int i = 0; i < 40; ++i) {
DUMMY_SOURCE.add(new ThreadLocal<CharSequence>());
}
}
private static final Set<Long> COUNTERS = new ConcurrentSkipListSet<Long>();
public static void main(String[] args) throws Exception {
final CountDownLatch startLatch = new CountDownLatch(THREADS_NUMBER);
final CountDownLatch endLatch = new CountDownLatch(THREADS_NUMBER);
for (int i = 0; i < THREADS_NUMBER; i++) {
new Thread() {
@Override
public void run() {
initDummyData();
startLatch.countDown();
try {
startLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
while (System.nanoTime() < END_TIME.get()) {
doJob();
}
COUNTERS.add(COUNTER.get().get());
DUMMY_COUNTER.addAndGet(DUMMY_DATA.get().get());
endLatch.countDown();
}
}.start();
}
startLatch.await();
END_TIME.set(System.nanoTime() + DURATION);
endLatch.await();
printStatistics();
}
private static void initDummyData() {
for (ThreadLocal<CharSequence> threadLocal : DUMMY_SOURCE) {
threadLocal.set(getRandomString());
}
}
private static CharSequence getRandomString() {
StringBuilder result = new StringBuilder();
Random random = new Random();
for (int i = 0; i < 127; ++i) {
result.append((char)random.nextInt(0xFF));
}
return result;
}
private static void doJob() {
Random random = new Random();
for (ThreadLocal<CharSequence> threadLocal : DUMMY_SOURCE) {
for (int i = 0; i < threadLocal.get().length(); ++i) {
DUMMY_DATA.get().addAndGet(threadLocal.get().charAt(i) << random.nextInt(31));
}
}
COUNTER.get().incrementAndGet();
}
private static void printStatistics() {
long total = 0L;
for (Long counter : COUNTERS) {
total += counter;
}
System.out.printf("Total iterations number: %d, dummy data: %d, distribution:%n", total, DUMMY_COUNTER.get());
for (Long counter : COUNTERS) {
System.out.printf("%f%%%n", counter * 100d / total);
}
}
}
I made four tests for two and ten thread scenarios and it shows performance loss is about 2.5% (78626 iterations for two threads and 76754 for ten threads), System resources are used by the threads approximately equally.
Also 'java.util.concurrent' authors suppose context switch time to be about 2000-4000 CPU cycles:
public class Exchanger<V> {
...
private static final int NCPU = Runtime.getRuntime().availableProcessors();
....
/**
* The number of times to spin (doing nothing except polling a
* memory location) before blocking or giving up while waiting to
* be fulfilled. Should be zero on uniprocessors. On
* multiprocessors, this value should be large enough so that two
* threads exchanging items as fast as possible block only when
* one of them is stalled (due to GC or preemption), but not much
* longer, to avoid wasting CPU resources. Seen differently, this
* value is a little over half the number of cycles of an average
* context switch time on most systems. The value here is
* approximately the average of those across a range of tested
* systems.
*/
private static final int SPINS = (NCPU == 1) ? 0 : 2000;