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# Debugging Memory Issues
This page is designed to help Chromium developers debug memory issues.
When in doubt, reach out to memory-dev@chromium.org.
[TOC]
## Investigating Reproducible Memory Issues
Let's say that there's a CL or feature that reproducibly increases memory usage
when it's landed/enabled, given a particular set of repro steps.
* Take a look at [the documentation](/docs/memory/README.md) for both
taking and navigating memory-infra traces.
* Take two memory-infra traces. One with the reproducible memory regression, and
one without.
* Load the memory-infra traces into two tabs.
* Compare the memory dump providers and look for the one that shows the
regression. Follow the relevant link.
* [The regression is in the Malloc MemoryDumpProvider.](#Investigating-Reproducible-Memory-Issues)
* [The regression is in a non-Malloc
MemoryDumpProvider.](#Regression-in-Non-Malloc-MemoryDumpProvider)
* [The regression is only observed in **private
footprint**.](#Regression-only-in-Private-Footprint)
* [No regression is observed.](#No-observed-regression)
### Regression in Malloc MemoryDumpProvider
Repeat the above steps, but this time also [take a heap
dump](#Taking-a-Heap-Dump). Confirm that the regression is also visible in the
heap dump, and then compare the two heap dumps to find the difference. You can
also use
[diff_heap_profiler.py](https://cs.chromium.org/chromium/src/third_party/catapult/experimental/tracing/bin/diff_heap_profiler.py)
to perform the diff.
### Regression in Non-Malloc MemoryDumpProvider
Hopefully the MemoryDumpProvider has sufficient information to help diagnose the
leak. Depending on the whether the leaked object is allocated via malloc or new
- it usually should be, you can also use the steps for debugging a Malloc
MemoryDumpProvider regression.
### Regression only in Private Footprint
* Repeat the repro steps, but instead of taking a memory-infra trace, use
the following tools to map the process's virtual space:
* On macOS, use vmmap
* On Windows, use SysInternal VMMap
* On other OSes, use /proc/<pid\>/smaps.
* The results should help diagnose what's happening. Contact the
memory-dev@chromium.org mailing list for more help.
### No observed regression
* If there isn't a regression in PrivateMemoryFootprint, then this might become
a question of semantics for what constitutes a memory regression. Common
problems include:
* Shared Memory, which is hard to attribute, but is mostly accounted for in
the memory-infra trace.
* Binary size, which is currently not accounted for anywhere.
## Investigating Heap Dumps From the Wild
For a small set of Chrome users in the wild, Chrome will record and upload
anonymized heap dumps. This has the benefit of wider coverage for real code
paths, at the expense of reproducibility.
These heap dumps can take some time to grok, but frequently yield valuable
insight. At the time of this writing, heap dumps from the wild have resulted in
real, high impact bugs being found in Chrome code ~90% of the time.
For an example investigation of a real heap dump, see [this
link](/docs/memory/investigating_heap_dump_example.md).
* Raw heap dumps can be viewed in the trace viewer. [See detailed
instructions.](/docs/memory-infra/heap_profiler.md#how-to-manually-browse-a-heap-dump).
This interface surfaces all available information, but can be overwhelming and
is usually unnecessary for investigating heap dumps.
* Important note: Heap profiling in the field uses
[poison process sampling](https://bugs.chromium.org/p/chromium/issues/detail?id=810748)
with a rate parameter of 10000. This means that for large/frequent allocations
[e.g. >100 MB], the noise will be quite small [much less than 1%]. But
there is noise so counts will not be exact.
* The heap dump summary typically contains all information necessary to diagnose
a memory issue.
* The stack trace of the potential memory leak is almost always sufficient to
tell the type of object being leaked, since most functions in Chrome
have a limited number of calls to new and malloc.
* The next thing to do is to determine whether the memory usage is intentional.
Very rarely, components in Chrome legitimately need to use many 100s of MBs of
memory. In this case, it's important to create a
[MemoryDumpProvider](https://cs.chromium.org/chromium/src/base/trace_event/memory_dump_provider.h)
to report this memory usage, so that we have a better understanding of which
components are using a lot of memory. For an example, see
[Issue 813046](https://bugs.chromium.org/p/chromium/issues/detail?id=813046).
* Assuming the memory usage is not intentional, the next thing to do is to
figure out what is causing the memory leak.
* The most common cause is adding elements to a container with no limit.
Usually the code makes assumptions about how frequently it will be called
in the wild, and something breaks those assumptions. Or sometimes the code
to clear the container is not called as frequently as expected [or at
all]. [Example
1](https://bugs.chromium.org/p/chromium/issues/detail?id=798012). [Example
2](https://bugs.chromium.org/p/chromium/issues/detail?id=804440).
* Retain cycles for ref-counted objects.
[Example](https://bugs.chromium.org/p/chromium/issues/detail?id=814334#c23)
* Straight up leaks resulting from incorrect use of APIs. [Example
1](https://bugs.chromium.org/p/chromium/issues/detail?id=801702#c31).
[Example
2](https://bugs.chromium.org/p/chromium/issues/detail?id=814444#c17).
## Taking a Heap Dump
Navigate to chrome://flags and search for **memlog**. There are several options
that can be used to configure heap dumps. All of these options are also
available as command line flags, for automated test runs [e.g. telemetry].
* `#memlog` controls which processes are profiled. It's also possible to
manually specify the process via the interface at `chrome://memory-internals`.
* `#memlog-sampling` will greatly reduce the overhead of the heap profiler, at
the expense of inaccuracy in small or infrequent allocations. Unless
performance is a concern, leave it disabled.
* `#memlog-stack-mode` describes the type of metadata recorded for each
allocation. `native` stacks provide the most utility. The only time the other
options should be considered is for Android official builds, most of which do
not support `native` stacks.
* `#memlog-keep-small-allocations` should be enabled, as it prevents the heap
dump exporter from pruning small allocations. Doing so yields smaller traces,
which is desirable when heap profiling is enabled in the wild.
Once the flags have been set appropriately, restart Chrome and take a
memory-infra trace. The results will have a heap dump.
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