Move C heap allocations into GC heap for RVALUE object data
Move C heap allocations into GC heap.
RVALUE is fixed width (40 bytes), some of which is used for accounting information and the remainder being used for storage of the object data (e.g. string contents). If the data required is larger than the available space inside the
RVALUE, memory is allocated outside of the GC heap using malloc and a pointer to the malloc’d region is stored inside the
We intend to remove this extra memory allocation in favour of storing the extra data in a contiguous region of slots in the heap that are adjacent to the original
The proposed memory layout changes in this feature branch can be seen in the following diagrams.
Original layout (master branch - with extra data being stored in malloc regions):
New layout (feature branch - with extra data being stored adjacent to it's owner in the GC heap):
Our hypothesis is that this feature will improve performance across Ruby by
- Improving locality, allowing better cache performance.
- Increased memory efficiency as fragmented data outside of GC can now be controlled and compacted by the GC.
- Reducing pointer indirection - as the extra object data (eg the Class
ivartable) is in a predictable place, we can find it by using the object address plus some offset rather than hopping over pointers.
- Eventually allowing us to remove the transient heap - The transient heap exists to reduce the number of these "extra data" malloc allocations. For new objects, the extra data is stored in a separate, pre-allocated heap, and then transferred to the C heap using
memcpyonce the data owner becomes an old object (survives 3 GC runs). This type of performance optimisation becomes redundant if we can store all object data in the eden heap.
This branch is the minimum viable implementation required to test our hypothesis. We introduce an API to allocate regions that are multiple slots wide in the heap and we implement this API for objects of type
T_CLASS so that the
rb_classext_t struct is stored in the heap alongside its owning
There are many things left to implement and some large things to fix. All of which will be addressed in future work, which we're working on at Shopify.
- We have consciously traded allocation performance for simplicity in this initial approach as read performance is where we think there will be a significant gain.
- GC Compaction is incompatible with this change. The current two finger algorithm is designed to support fixed width memory regions. When an object occupies multiple contiguous slots this algorithm no longer works.
- Incremental marking is disabled when this feature is enabled. This is because we are unable to predict the allocation pattern in each mark step now that allocations can be multiple slots wide.
- We cannot currently resize regions. For example, when mutating strings, we currently
reallocone region to a new larger region, we will support resizing when all object data is in the eden heap.
- This API is currently only implemented for
T_CLASSobjects, we need to implement it for all other Ruby types to gain the most benefit.
We believe this feature is useful enough that we'd like to merge this PR. We would like feedback from the Core team and the community.
Merging this feature will also allow us to continue working without maintaining a long lived fork of Ruby.
There will be no negative effects of merging this PR as all functionality is disabled by default.
How to enable this feature¶
This feature must be enabled at compile time with the flag
cppflags='-DUSE_RVARGC=1' ./configure make clean && make && make install
We ran the following snippet to generate a heap dump and then used a separate script to visualise it.
require "objspace" ObjectSpace.dump_all(output: File.open("heap.json", "w"), full: true)
In the following output each 2x2 pixel region represents a single slot in a heap page. Heap pages are organised vertically and from low to high memory address.
- Blue pixels represents
- Yellow pixels represents
- Grey pixels are other live objects
So we can see that every
T_CLASS is now followed by 3
T_PAYLOAD objects - this is because
sizeof(rb_classext_t) == 104 and the space available in 3
T_PAYLOAD slots is
112 (we require 8 bytes for bookkeeping).
We prepared a simple micro-benchmark in order to measure the speed of
ivar access and method calls, as references to the method table and the ivar table are stored inside
rb_classext_struct, which is always allocated on the malloc heap.
require 'benchmark/ips' class Foobar def initialize @foo = 3 end def foo @foo self end def bar @foo + 1 self end def baz @foo + 2 self end end @f = Foobar.new Benchmark.ips do |x| x.report("method & ivar lookup") do |times| i = 0 while i < times @f.foo.bar.baz i += 1 end end end
We ran this 5 times on our branch and took the scores, and then again on master (at 32b7dcfb56a417c1d1c354102351fc1825d653bf - the base of our feature branch) and took the scores.
This graph shows the mean of our results combined with the standard error range for each set of samples.
Micro-benchmark: Million iterations per second, higher is better master: 16.372 16.399 16.058 15.98 16.426 feature: 16.901 16.909 16.695 16.757 16.818
This shows a ~3% improvement in our feature-branch over master on average, with the worst case (upper bound of standard error on master vs lower bound standard error on our feature branch) showing a ~2% improvement.
We also ran the optcarrot benchmark on each branch and the results were as follows:
Optcarrot: Frames per second, higher is better master: 43.27430926 42.65007047 43.32562887 42.64047665 43.40328494 feature: 41.59069356 40.51252649 40.55757381 41.76160937 42.95645122
This shows a that our branch is ~4% slower than master (with the branch upper vs master lower standard error difference being ~2%).
Raw benchmark numbers and the script used for generating these charts is in this gist.
Despite the Optcarrot performance drop we feel that these results are promising for the future of this work - taking into consideration our significantly worse allocation performance (improving allocation performance is a high priority part of our roadmap).
Updated by eightbitraptor (Matthew Valentine-House) about 1 month ago
nobu (Nobuyoshi Nakada) wrote in #note-2:
Thank you for the great work.
It is interesting idea.
At first glance, “GC Compaction” and “Incremental marking” seem major drawbacks, I’ll see the patch more.
Thank you for taking the time to look.
We're definitely aware that the impact to Compaction and incremental marking are major drawbacks. We will address those as a future part of this work. We pushed this knowing about the drawbacks so that we could make other developers aware of our intention and get some feedback on the idea.