wenbingshen commented on code in PR #3417:
URL: https://github.com/apache/bookkeeper/pull/3417#discussion_r929570822
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bookkeeper-server/src/main/java/org/apache/bookkeeper/util/collections/ConcurrentLongHashMap.java:
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@@ -507,7 +507,11 @@ private V remove(long key, Object value, int keyHash) {
} finally {
if (autoShrink && size < resizeThresholdBelow) {
try {
- int newCapacity = alignToPowerOfTwo((int) (capacity /
shrinkFactor));
+ // Shrinking must at least ensure initCapacity,
+ // so as to avoid frequent shrinking and expansion
near initCapacity,
+ // frequent shrinking and expansion,
+ // additionally opened arrays will consume more memory
and affect GC
+ int newCapacity = Math.max(alignToPowerOfTwo((int)
(capacity / shrinkFactor)), initCapacity);
Review Comment:
Assuming that `initCapacity` is 256,`mapFillFactor, mapIdleFactor,
expandFactor, shrinkFactor ` keep the default values:
| initCapacity | capacity | mapFillFactor | mapIdleFactor |
resizeThresholdUp | resizeThresholdBelow | expandFactor | shrinkFactor |
| ----------- | -------- | ------------- | ------------- |
----------------- | -------------------- | ------------ | ------------ |
| 256 | 256 | 0.66f | 0.15f | 168
| 38 | 2 | 2 |
| 256 | 512 | 0.66f | 0.15f | 337
| 76 | 2 | 2 |
| 256 | 1024 | 0.66f | 0.15f | 675
| 153 | 2 | 2 |
| 256 | 2048 | 0.66f | 0.15f | 1351
| 307 | 2 | 2 |
When the `capacity` is expanded to 1024, the `size` needs to be less than
153 to shrink, and size=153 is less than initCapacity=256, causing at least
1024-153 of space wasted. This waste is worse when initCapacity is larger.
Therefore, we cannot use `size` to judge that it will not shrink when it is
smaller than `initCapacity`.
@zymap PTAL
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