--heapsnapshot-near-heap-limit=max_count
当 V8 堆使用量接近堆限制时，将 V8 堆快照写入磁盘。
count 应该是非负整数（在这种情况下，Node.js 将不超过 max_count 的快照写入磁盘）。
当生成快照时，可能会触发垃圾回收并降低堆使用率。
因此，在 Node.js 实例最终内存不足之前，可能会将多个快照写入磁盘。
可以比较这些堆快照以确定在拍摄连续快照期间正在分配哪些对象。
不能保证 Node.js 会准确地将 max_count 个快照写入磁盘，但是当 max_count 大于 0 时，它会尽量在 Node.js 实例耗尽内存之前生成至少一个和最多 max_count 个快照。
生成 V8 快照需要时间和内存（由 V8 堆管理的内存和 V8 堆外的原生内存）。 堆越大，需要的资源越多。 Node.js 会调整 V8 堆以适应额外的 V8 堆内存开销，并尽量避免耗尽进程可用的所有内存。 当进程使用的内存超过系统认为合适的内存时，该进程可能会被系统突然终止，具体取决于系统配置。
$ node --max-old-space-size=100 --heapsnapshot-near-heap-limit=3 index.js
Wrote snapshot to Heap.20200430.100036.49580.0.001.heapsnapshot
Wrote snapshot to Heap.20200430.100037.49580.0.002.heapsnapshot
Wrote snapshot to Heap.20200430.100038.49580.0.003.heapsnapshot
<--- Last few GCs --->
[49580:0x110000000] 4826 ms: Mark-sweep 130.6 (147.8) -> 130.5 (147.8) MB, 27.4 / 0.0 ms (average mu = 0.126, current mu = 0.034) allocation failure scavenge might not succeed
[49580:0x110000000] 4845 ms: Mark-sweep 130.6 (147.8) -> 130.6 (147.8) MB, 18.8 / 0.0 ms (average mu = 0.088, current mu = 0.031) allocation failure scavenge might not succeed
<--- JS stacktrace --->
FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
....Writes a V8 heap snapshot to disk when the V8 heap usage is approaching the
heap limit. count should be a non-negative integer (in which case
Node.js will write no more than max_count snapshots to disk).
When generating snapshots, garbage collection may be triggered and bring
the heap usage down. Therefore multiple snapshots may be written to disk
before the Node.js instance finally runs out of memory. These heap snapshots
can be compared to determine what objects are being allocated during the
time consecutive snapshots are taken. It's not guaranteed that Node.js will
write exactly max_count snapshots to disk, but it will try
its best to generate at least one and up to max_count snapshots before the
Node.js instance runs out of memory when max_count is greater than 0.
Generating V8 snapshots takes time and memory (both memory managed by the V8 heap and native memory outside the V8 heap). The bigger the heap is, the more resources it needs. Node.js will adjust the V8 heap to accommodate the additional V8 heap memory overhead, and try its best to avoid using up all the memory available to the process. When the process uses more memory than the system deems appropriate, the process may be terminated abruptly by the system, depending on the system configuration.
$ node --max-old-space-size=100 --heapsnapshot-near-heap-limit=3 index.js
Wrote snapshot to Heap.20200430.100036.49580.0.001.heapsnapshot
Wrote snapshot to Heap.20200430.100037.49580.0.002.heapsnapshot
Wrote snapshot to Heap.20200430.100038.49580.0.003.heapsnapshot
<--- Last few GCs --->
[49580:0x110000000] 4826 ms: Mark-sweep 130.6 (147.8) -> 130.5 (147.8) MB, 27.4 / 0.0 ms (average mu = 0.126, current mu = 0.034) allocation failure scavenge might not succeed
[49580:0x110000000] 4845 ms: Mark-sweep 130.6 (147.8) -> 130.6 (147.8) MB, 18.8 / 0.0 ms (average mu = 0.088, current mu = 0.031) allocation failure scavenge might not succeed
<--- JS stacktrace --->
FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
....