On 2010-02-02, at 7:49 PM, Bryan Thompson wrote:
Vladimir,
I now have a workload where I have a
hotspot on our cache which is 8% of the total time. I am going to use
this to test BCHM under a realistic workload.
One thing which I like about your
design is that by placing the array buffering the operations for a thread
within the Segment, you are using the lock guarding the Segment to control
updates to that array. While this has less potential throughput than a
truely thread-local design (which is non-blocking until the thread-local
array is full), it seems that the array buffering the updates can not
"escape" under your design. Was this intentional?
Yes. Escaping could also potentially be handled by shared pool of array
buffers. Ideally shared pool would be some lock free structure since
there would be a lot of contention among threads for these array buffers
(that record accesses)
The danger with a true thread-local
design is that a thread can come in and do some work, get some updates
buffered in its thread-local array, and then never visit again. In
this case those updates would remain buffered on the thread and would not in
fact cause the access order to be updated in a timely manner. Worse,
if you are relying on WeakReference semantics, the buffered updates would
remain strongly reachable and the corresponding objects would be wired into
the cache.
Not if you return updates into above mentioned pool as the thread unwinds
from call into cache (container).
I've worked around this issue in a
different context where it made sense to scope the buffers to an owning
object (a B+Tree instance). In that case, when the B+Tree container
was closed, all buffered updates were discarded. This nicely
eliminated the problems with "escaping"
threads.
Yes, there
has to be some container boundary! In Infinispan this would be a cache
instance, DataContainer to be exact.
However, I like your approach
better.
Bryan
PS: I see a comment on LRU#onEntryHit() indicating that it is invoked
without holding the Segment lock. Is that true only when BCHM#get() is
invoked?
Yes, I'll change javadoc to say : "is potentially invoked without holding
a lock".
PPS: Also, can you expand on the role of the LRU#accessQueue vs
LRU#lruQueue? Is there anything which corresponds to the total LRU
order (after batching updates through the
Segment)?
LRU#accessQueue records hits on a Segment. This is the batching FIFO
queue from BP-Wrapper paper except it is not per thread but rather per
Segment. Notice that LRU#accessQueue is lock free but thread safe since we
expect a lot of threads recording accesses per Segment. Ideal collection for
this use case is ConcurrentLinkedQueue.
LRU#lruQueue is simply just a simple LRU stack implementation with one
end of the queue holding most recently accessed entries and the other end of
the queue with least recently accessed entries.
Regards,
Vladimir