Why PoolThreadLocalCache class initialValue method use synchronized ???

final class PoolThreadLocalCache extends FastThreadLocal<PoolThreadCache> {
        private final boolean useCacheForAllThreads;

        PoolThreadLocalCache(boolean useCacheForAllThreads) {
            this.useCacheForAllThreads = useCacheForAllThreads;
        }

        @Override
        protected synchronized PoolThreadCache initialValue() {
            final PoolArena<byte[]> heapArena = leastUsedArena(heapArenas);
            final PoolArena<ByteBuffer> directArena = leastUsedArena(directArenas);

            final Thread current = Thread.currentThread();
            if (useCacheForAllThreads || current instanceof FastThreadLocalThread) {
                final PoolThreadCache cache = new PoolThreadCache(
                        heapArena, directArena, tinyCacheSize, smallCacheSize, normalCacheSize,
                        DEFAULT_MAX_CACHED_BUFFER_CAPACITY, DEFAULT_CACHE_TRIM_INTERVAL);

                if (DEFAULT_CACHE_TRIM_INTERVAL_MILLIS > 0) {
                    final EventExecutor executor = ThreadExecutorMap.currentExecutor();
                    if (executor != null) {
                        executor.scheduleAtFixedRate(trimTask, DEFAULT_CACHE_TRIM_INTERVAL_MILLIS,
                                DEFAULT_CACHE_TRIM_INTERVAL_MILLIS, TimeUnit.MILLISECONDS);
                    }
                }
                return cache;
            }
            // No caching so just use 0 as sizes.
            return new PoolThreadCache(heapArena, directArena, 0, 0, 0, 0, 0);
        }

I think that’s unnecessary， initialValue method use synchronized ???

[jiangxinlingdu]

@normanmaurer，https://github.com/netty/netty/issues/9284 About FastThreadLocal Cache line padding I haven’t figured it out yet， hope you can explain. Thank you

[normanmaurer]

@jiangxinlingdu thanks a lot! https://github.com/netty/netty/pull/9357