RFR: 8259886 : Improve SSL session cache performance and scalability [v2]

[djelinski]

On Thu, 4 Feb 2021 20:45:55 GMT, djelinski <github.com+30433125+djelinski at openjdk.org> wrote:

>> Thank you for the comment. The big picture is more clear to me now.
>> 
>>> Example 2:
>>> Old implementation will get:
>>> |K=3, exp=10|K=5, exp=12|K=7, exp=14|K=9, exp=16|
>>>
>>> New implementation will get:
>>> |K=5, exp=12|K=7, exp=14|K=1, exp=8(expired)|K=9, exp=16|
>> 
>> K=3 is not expired yet, but get removed, while K=1 is kept.  This behavior change may cause more overall performance hurt than improving the cache put/get performance.  For example, it need to grab the value remotely.   A full handshake or OCSP status grabbing could counteract all the performance gain with the cache update.
>> 
>>> All calls to put() remove expired items from the front of the queue, and never perform a full scan. get() calls shuffle the queue, moving the accessed item to the back. Compare this to original code where put() only removed expired items when the cache overflowed, and scanned the entire cache.
>> 
>> I think the idea that put() remove expired items from the front of the queue is good.  I was wondering if it is an option to have the get() method that removed expired items until the 1st un-expired item, without scan the full queue and change the order of the queue.  But there is still an issue that the SoftReference may have clear an item, which may be still valid.
>> 
>> In general, I think the get() performance is more important than put() method, as get() is called more frequently.  So we should try to keep the cache small if possible.
>> 
>>>> increase the size to some big scales, like 2M and 20M
>>>
>>> Can do. Do you think it makes sense to also benchmark the scenario where GC kicks in and collects soft references?
>> 
>> In the update, the SoftReference.clear() get removed.  I'm not sure of the impact of the enqueued objects any longer.  In theory, it could improve the memory use, which could counteract the performance gain in some situation.
>> 
>>> Also, what do you think about the changes done in Do not invalidate objects before GC 5859a03 commit? 
>> 
>> See above, it is a concern to me that the soft reference cannot be cleared with this update.
>> 
>>> How do I file a CSR?
>> 
>> Could you edit the bug: https://bugs.openjdk.java.net/browse/JDK-8259886?  In the more drop down menu, there is a "Create CSR" option.  You can do it if we have an agreement about the solution and impact.
>
> Thanks for your review! Some comments below.
>> A full handshake or OCSP status grabbing could counteract all the performance gain with the cache update.
> 
> Yes, but that's unlikely. Note that K=3 is before K=1 in the queue only because 3 wasn't used since 1 was last used. This means that either K=3 is used less frequently than K=1, or that all cached items are in active use. In the former case we don't lose much by dropping K=3 (granted, there's nothing to offset that). In the latter we are dealing with full cache at all times, which means that most `put()`s would scan the queue, and we will gain a lot by finishing faster.
>> get() [..] without [..] change the order of the queue
> 
> If we do that, frequently used entries will be evicted at the same age as never used ones. This means we will have to recompute (full handshake/fresh OCSP) both the frequently used and the infrequently used entries. It's better to recompute only the infrequently used ones, and reuse the frequently used as long as possible - we will do less work that way.
> That's probably the reason why a `LinkedHashMap` with `accessOrder=true` was chosen as the backing store implementation originally.
>> get() performance is more important [..] so we should try to keep the cache small if possible
> 
> I don't see the link; could you explain?
>> In the update, the SoftReference.clear() get removed. I'm not sure of the impact of the enqueued objects any longer. In theory, it could improve the memory use, which could counteract the performance gain in some situation.
> 
> That's the best part: no objects ever get enqueued! We only called `clear()` right before losing the last reference to `SoftCacheEntry` (which is the `SoftReference`). When GC collects the `SoftReference`, it does not enqueue anything. GC only enqueues the `SoftReference` when it collects the referenced object (session / OCSP response) without collecting the `SoftReference` (cache entry) itself.
> This is [documented behavior](https://docs.oracle.com/javase/7/docs/api/java/lang/ref/package-summary.html): _If a registered reference becomes unreachable itself, then it will never be enqueued._
>> Could you edit the bug
> 
> I'd need an account on the bug tracker first.

So, how do we want to proceed here? Is the proposed solution acceptable? If not, what needs to change? if yes, what do I need to do next?

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PR: https://git.openjdk.java.net/jdk/pull/2255