How to detect v8 heap usage close to limit in Node.js

Question

Now I use:

const v8 = require('v8'); let heap = v8.getHeapStatistics(); let usage = 100 / heap.heap_size_limit * heap.used_heap_size; if (usage > 90) { console.log(`V8 heap usage close to the limit (${usage.toFixed()}%)`); } else if (usage > 95) { console.log(`V8 heap usage very close to the limit (${usage.toFixed()}%)`); } 

This solution does not work properly.

When I try this command: node --max-old-space-size = 100 index.js My process cannot allocate memory when my script calculates ~ 56%

 getHeapStatistics().heap_size_limit is 178 MB getHeapStatistics().used_heap_size is ~95 MB 

How to determine the situation more accurately when we can get rid of a memory error?

Answer 1

The v8 heap is actually divided into regions or spaces: new_space, old_space, code_space, map_space, and large_object_space. The first two roughly correspond to the nursery and a bunch of objects, although in theory objects can be in any of the five spaces.

A more accurate measure would be to use space_size and space_used_size of the old_space.

 var space = v8.getHeapSpaceStatistics(); var old_space_total = space[1].space_size; var old_space_used = space[1].space_used_size; var usage = 100 / old_space_total * old_space_used; if (usage > 90) { console.log(`V8 heap usage close to the limit (${usage.toFixed()}%)`); } else if (usage > 95) { console.log(`V8 heap usage very close to the limit (${usage.toFixed()}%)`); } 

Statistics immediately before running out of memory:

 #node --max_old_space_size=200 heap.js ... { total_heap_size: 243306496, total_heap_size_executable: 5242880, total_physical_size: 243306496, total_available_size: 19149632, used_heap_size: 215801616, heap_size_limit: 243269632 } [ { space_name: 'new_space', space_size: 33554432, space_used_size: 16464888, space_available_size: 41992, physical_space_size: 33554432 }, { space_name: 'old_space', space_size: 205475840, space_used_size: 197215464, space_available_size: 18462512, physical_space_size: 205475840 }, { space_name: 'code_space', space_size: 2097152, space_used_size: 680416, space_available_size: 617024, physical_space_size: 2097152 }, { space_name: 'map_space', space_size: 2179072, space_used_size: 1452528, space_available_size: 18800, physical_space_size: 2179072 }, { space_name: 'large_object_space', space_size: 0, space_used_size: 0, space_available_size: 0, physical_space_size: 0 } ] V8 heap usage close to the limit (96%) ... FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory 

As we can see, total_heap_size: 243306496 is actually a summation of the new space (33554432), old space (205475840), code (2097152), map (2179072), large object (0).

And as you can see, all other spaces are healthy, and the old space is approaching exhaustion, and from there comes true exhaustion.

Hope this helps.