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Increase default 'max_semi_space_size' value to reduce GC overhead in V8 #42511
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@nodejs/v8
I remember there were similar requests for
max_old_space_size(whose relatively small value is the source of a lot of confusion) - if we decide to use different defaults, I suppose these should all be tweaked to an optimized setting, though it's difficult to tell which setting is the best as a one-size-fits-all solution and I think that's why we've been keeping the V8 defaults so far (and another factor is that these are all configurable via the command line and in general Node.js core prefers to let the user choose what's best for their use cases instead of picking one itself)@joyeecheung Thanks for your explanation! Is there any official documents about the Node.js core prefers? And If there are
some documents, can we put this preferablemax_semi_space_sizeconfiguration in it?I guess this also applies to workers and the
maxYoungGenerationSizeMboption?Actually it doesn't seem to be possible to set semi space for workers? @jasnell @addaleax
Only:
- max_young
- max_old
Are possible to set. https://lbwa.github.io/v8-reference/classv8_1_1_resource_constraints.html
and another factor is that these are all configurable via the command line and in general Node.js core prefers to let the user choose what's best for their use cases instead of picking one itself
The problem with this is that 99% of users don't know what is best for their use case so I guess we have to make some kind of decision in terms of defaults that works best for most of our users. I guess the current defaults are more optimized for browser workloads rather than server workloads? IMHO it might be worth looking into optimizing the defaults to better suit our users.
And If there are some documents, can we put this preferable max_semi_space_size configuration in it?
We already document
--max-old-space-sizewith some suggestions (https://github.com/nodejs/node/blob/master/doc/api/cli.md#--max-old-space-sizesize-in-megabytes), so adding documentation for the semi space size there makes sense, too.Reacted by leslie, Joe Bowbeer and Jiabin PengThe problem with this is that 99% of users don't know what is best for their use case.
I agree with this, and V8 should consider the browser's memory consumption in mobile device with small RAM size. But for server scenarios, memory usually isn't the bottleneck.
V8 has provided the interface set_max_semi_space_size_in_kb() to change the default max_semi_space_size. I found a related issue try to setup the default max_yong and max_old generation size according to system's physical_memory, which uses the ResourceConstraints::ConfigureDefaults interface in V8. But V8 has limited the max heap size as 2GB, and there is a proportional relationship between semi_space_size and heap_size, so the default max_semi_space_size can't large than 16MB. Can we create a similar
ConfigureDefaultsfunction for node and choose an optimal configuration for server scenarios?- added a commit that references this issue
on Apr 2, 2022 We already document
--max-old-space-sizewith some suggestions (https://github.com/nodejs/node/blob/master/doc/api/cli.md#--max-old-space-sizesize-in-megabytes), so adding documentation for the semi space size there makes sense, too.OK, I added the
--max_semi_space_sizeintroduction into the document, this is the related PR (#42575).17 remaining items
Context: #46608 (comment)
Was discussed on TSC meeting again. There is interest on seeing a PR. Until then there is not much to discuss on TSC level.
- added a commit that references this issue
on Aug 22, 2023 There has been no activity on this feature request for 5 months and it is unlikely to be implemented. It will be closed 6 months after the last non-automated comment.
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I guess this issue should stay open as a reminder for making a PR?
There's nodejs/performance#67 already. I'm going to close this but feel free to send a PR.
What is the problem this feature will solve?
When I use node to run the web-tooling-benchmark, I found that the runtime flag
--max_semi_space_sizehave a big impact on the test result. The total throughput increased about 18% after I pass the runtime flag--max_semi_space_size=128into node. So I did some investigate about the 'max_semi_space_size' flag.From some v8 official blogs (Getting garbage collection for free, orinoco-parallel-scavenger), I found there are two garbage collection strategies in V8:
When we create a new object by javascript code, the object will be put into semi_space as a young generation object. And when the semi_space is about to use up, V8 engine will trigger the Scavenge GC to clean up the garbage objects in semi_space.
If I use the
--max_semi_space_sizeflag to increase the maximum limit of semi_space size, the scavenge GC occur frequency will decrease. This will bring both advantages and disadvantages:It's a trade-off between time and space. V8 set the default
max_semi_space_sizeas 16MB for 64bit system and 8MB for 32bit system (related code). I think it's a heuristic value that mainly considered client device with small RAM size (for example: some android device only have 4GB RMA). But for server scenarios, memory usually isn't the bottleneck, while throughput is the actual bottleneck.So the problem is:
Whether the currently default
max_semi_space_size(16MB/8MB) for V8 is also the best configuration for node?What is the feature you are proposing to solve the problem?
To solve the problem above, I tuned the
--max_semi_space_size(16MB, 32MB, 64MB, 128MB, 256MB) and tested on web-tooling-benchmark and a simple service based on ghost.js. Here is the test results:From the figure we can see that:
So I think we can choose a better
max_semi_space_sizevalue and pass this runtime flag to V8 when node startup.What alternatives have you considered?
Test environment:
Test process: