Re: multithreaded cache?
On 5/15/2012 5:14 AM, bugbear wrote:
I'm using various Apache Commons Maps as a
multithread cache, protected using
ReentrantReadWriteLock, so that getting() uses a read lock,
and putting() uses a write lock.
But I've got an issue; in the
case of a cache miss (protected by a read lock),
the required value is acquired using the "underlying function"
that the cache is over; this value is then put() into
the cache (protected by a write lock)
This is all perfectly thread safe, and gives
correct results.
However, if the underlying function is slow
and/or resource hungry (consider cacheing
a ray traced image!) many threads can
end up calling the real function (second
and subsequent threads to the first get a miss
during the first threads call to the underlying function).
"obviously..." what I want is for only
the thread that FIRST has a cache miss
calls the underlying function, whilst other
threads (for the same key) wait.
This seems "obvious" enough that I'm guessing
there's a standard solution.
Googling led me to several "heavy" libraries;
This appears more a locking/cacheing issue
than a Java issue, although my implementation
is in Java.
Can anyone (please) point me at a
canonical solution/implementation?
Don't know whether it's "canonical," but one fairly obvious
approach is to insert the missed key in the map immediately, but
with a value that means "Coming Soon To A Cache Near You." The
original thread can then release its read lock and fetch the true
value from its source.
Other threads that look for the same value will "hit" the
cache, but will recognize the "Coming Soon" as an indication to
wait until the first thread comes up with the value.
There must be many ways to arrange this, but one that requires
very little awareness on the part of the querying threads is to
make the hashed "value" a value-holder object. Then a querying
thread could do something like
mapLock.lockForReading();
ValueHolder holder = map.get(key);
mapLock.release();
if (holder == null) {
mapLock.lockForWriting();
holder = map.get(key); // inserted by other thread?
if (holder == null) {
holder = new ValueHolder(key);
map.put(key, holder);
}
mapLock.release();
}
Value = holder.getValue();
The ValueHolder class might look like
class ValueHolder {
private final Key key;
private Value cachedValue;
private SourceException fetchError;
ValueHolder(Key key) {
this.key = key;
}
synchronized
Value getValue() throws SourceException {
if (cachedValue == null) {
if (fetchError != null) {
// notify new thread of old error
throw fetchError;
}
try {
cachedValue = fetchValueFromSource(key);
} catch (SourceException ex) {
fetchError = ex;
throw fetchError;
}
}
return cachedValue;
}
}
(That's not the only way to deal with errors in fetching, but it
seems perfectly reasonable to me even if it's a little odd to throw
the same exception object multiple times.)
The cache-querying threads lock the map only briefly, long
enough to determine whether a ValueHolder is present. ValueHolder
can delay its callers for a long time (if not, why cache at all?),
but will only delay callers that are interested in that particular
key; callers looking for other keys will get different ValueHolder
instances and won't be interfered with. And only one thread will
actually attempt fetchValueFromSource() on any given key.
--
Eric Sosman
esosman@ieee-dot-org.invalid