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# Bloatview
BloatView is a tool that shows information about cumulative memory usage
and leaks. If it finds leaks, you can use [refcount tracing and balancing](refcount_tracing_and_balancing.md)
to discover the root cause.
## How to build with BloatView
Build with `--enable-debug` or `--enable-logrefcnt`.
## How to run with BloatView
The are two environment variables that can be used.
XPCOM_MEM_BLOAT_LOG
If set, this causes a *bloat log* to be printed on program exit, and
each time `nsTraceRefcnt::DumpStatistics` is called. This log contains
data on leaks and bloat (a.k.a. usage).
XPCOM_MEM_LEAK_LOG
This is similar to `XPCOM_MEM_BLOAT_LOG`, but restricts the log to only
show data on leaks.
You can set these environment variables to any of the following values.
- **1** - log to stdout.
- **2** - log to stderr.
- ***filename*** - write log to a file.
## Reading individual bloat logs
Full BloatView output contains per-class statistics on allocations and
refcounts, and provides gross numbers on the amount of memory being
leaked broken down by class. Here's a sample of the BloatView output.
== BloatView: ALL (cumulative) LEAK AND BLOAT STATISTICS, tab process 1862
|<----------------Class--------------->|<-----Bytes------>|<----Objects---->|
| | Per-Inst Leaked| Total Rem|
0 |TOTAL | 17 2484|253953338 38|
17 |AsyncTransactionTrackersHolder | 40 40| 10594 1|
78 |CompositorChild | 472 472| 1 1|
79 |CondVar | 24 48| 3086 2|
279 |MessagePump | 8 8| 30 1|
285 |Mutex | 20 60| 89987 3|
302 |PCompositorChild | 412 412| 1 1|
308 |PImageBridgeChild | 416 416| 1 1|
The first line tells you the pid of the leaking process, along with the
type of process.
Here's how you interpret the columns.
- The first, numerical column [is the index](https://searchfox.org/mozilla-central/source/xpcom/base/nsTraceRefcnt.cpp#365)
of the leaking class.
- **Class** - The name of the class in question (truncated to 20
characters).
- **Bytes Per-Inst** - The number of bytes returned if you were to
write `sizeof(Class)`. Note that this number does not reflect any
memory held onto by the class, such as internal buffers, etc. (E.g.
for `nsString` you'll see the size of the header struct, not the
size of the string contents!)
- **Bytes Leaked** - The number of bytes per instance times the number
of objects leaked: (Bytes Per-Inst) x (Objects Rem). Use this number
to look for the worst offenders. (Should be zero!)
- **Objects Total** - The total count of objects allocated of a given
class.
- **Objects Rem** - The number of objects allocated of a given class
that weren't deleted. (Should be zero!)
Interesting things to look for:
- **Are your classes in the list?** - Look! If they aren't, then
you're not using the `NS_IMPL_ADDREF` and `NS_IMPL_RELEASE` (or
`NS_IMPL_ISUPPORTS` which calls them) for xpcom objects, or
`MOZ_COUNT_CTOR` and `MOZ_COUNT_DTOR` for non-xpcom objects. Not
having your classes in the list is *not* ok. That means no one is
looking at them, and we won't be able to tell if someone introduces
a leak. (See
[below](#how-to-instrument-your-objects-for-bloatview)
for how to fix this.)
- **The Bytes Leaked for your classes should be zero!** - Need I say
more? If it isn't, you should use the other tools to fix it.
- **The number of objects remaining might not be equal to the total
number of objects.** This could indicate a hand-written Release
method (that doesn't use the `NS_LOG_RELEASE` macro from
nsTraceRefcnt.h), or perhaps you're just not freeing any of the
instances you've allocated. These sorts of leaks are easy to fix.
- **The total number of objects might be 1.** This might indicate a
global variable or service. Usually this will have a large number of
refcounts.
If you find leaks, you can use [refcount tracing and balancing](refcount_tracing_and_balancing.md)
to discover the root cause.
## Combining and sorting bloat logs
You can view one or more bloat logs in your browser by running the
following program.
perl tools/bloatview/bloattable.pl *log1* *log2* \... *logn* >
*htmlfile*
This will produce an HTML file that contains a table similar to the
following (but with added JavaScript so you can sort the data by
column).
Byte Bloats
---------- ---------------- --------------------------
Name File Date
blank `blank.txt` Tue Aug 29 14:17:40 2000
mozilla `mozilla.txt` Tue Aug 29 14:18:42 2000
yahoo `yahoo.txt` Tue Aug 29 14:19:32 2000
netscape `netscape.txt` Tue Aug 29 14:20:14 2000
---------- ---------------- --------------------------
The numbers do not include malloc'd data such as string contents.
Click on a column heading to sort by that column. Click on a class name
to see details for that class.
-------------------- --------------- ----------------- --------- --------- ---------- ---------- ------------------------------- --------- -------- ---------- ---------
Class Name Instance Size Bytes allocated Bytes allocated but not freed
blank mozilla yahoo netscape Total blank mozilla yahoo netscape Total
TOTAL 1754408 432556 179828 404184 2770976
nsStr 20 6261600 3781900 1120920 1791340 12955760 222760 48760 13280 76160 360960
nsHashKey 8 610568 1842400 2457872 1134592 6045432 32000 536 568 1216 34320
nsTextTransformer 548 8220 469088 1414936 1532756 3425000 0 0 0 0 0
nsStyleContextData 736 259808 325312 489440 338560 1413120 141312 220800 -11040 94944 446016
nsLineLayout 1100 2200 225500 402600 562100 1192400 0 0 0 0 0
nsLocalFile 424 558832 19928 1696 1272 581728 72080 1272 424 -424 73352
-------------------- --------------- ----------------- --------- --------- ---------- ---------- ------------------------------- --------- -------- ---------- ---------
The first set of columns, **Bytes allocated**, shows the amount of
memory allocated for the first log file (`blank.txt`), the difference
between the first log file and the second (`mozilla.txt`), the
difference between the second log file and the third (`yahoo.txt`), the
difference between the third log file and the fourth (`netscape.txt`),
and the total amount of memory allocated in the fourth log file. These
columns provide an idea of how hard the memory allocator has to work,
but they do not indicate the size of the working set.
The second set of columns, **Bytes allocated but not freed**, shows the
net memory gain or loss by subtracting the amount of memory freed from
the amount allocated.
The **Show Objects** and **Show References** buttons show the same
statistics but counting objects or `AddRef`'d references rather than
bytes.
## Comparing Bloat Logs
You can also compare any two bloat logs (either those produced when the
program shuts down, or written to the bloatlogs directory) by running
the following program.
`perl tools/bloatview/bloatdiff.pl` <previous-log> <current-log>
This will give you output of the form:
Bloat/Leak Delta Report
Current file: dist/win32_D.OBJ/bin/bloatlogs/all-1999-10-22-133450.txt
Previous file: dist/win32_D.OBJ/bin/bloatlogs/all-1999-10-16-010302.txt
--------------------------------------------------------------------------
CLASS LEAKS delta BLOAT delta
--------------------------------------------------------------------------
TOTAL 6113530 2.79% 67064808 9.18%
StyleContextImpl 265440 81.19% 283584 -26.99%
CToken 236500 17.32% 306676 20.64%
nsStr 217760 14.94% 5817060 7.63%
nsXULAttribute 113048 -70.92% 113568 -71.16%
LiteralImpl 53280 26.62% 75840 19.40%
nsXULElement 51648 0.00% 51648 0.00%
nsProfile 51224 0.00% 51224 0.00%
nsFrame 47568 -26.15% 48096 -50.49%
CSSDeclarationImpl 42984 0.67% 43488 0.67%
This "delta report" shows the leak offenders, sorted from most leaks
to fewest. The delta numbers show the percentage change between runs for
the amount of leaks and amount of bloat (negative numbers are better!).
The bloat number is a metric determined by multiplying the total number
of objects allocated of a given class by the class size. Note that
although this isn't necessarily the amount of memory consumed at any
given time, it does give an indication of how much memory we're
consuming. The more memory in general, the worse the performance and
footprint. The percentage 99999.99% will show up indicating an
"infinite" amount of leakage. This happens when something that didn't
leak before is now leaking.
## BloatView and continuous integration
BloatView runs on debug builds for many of the test suites Mozilla has
running under continuous integration. If a new leak occurs, it will
trigger a test job failure.
BloatView's output file can also show you where the leaked objects are
allocated. To do so, the `XPCOM_MEM_LOG_CLASSES` environment variable
should be set to the name of the class from the BloatView table:
XPCOM_MEM_LOG_CLASSES=MyClass mach mochitest [options]
Multiple class names can be specified by setting `XPCOM_MEM_LOG_CLASSES`
to a comma-separated list of names:
XPCOM_MEM_LOG_CLASSES=MyClass,MyOtherClass,DeliberatelyLeakedClass mach mochitest [options]
Test harness scripts typically accept a `--setenv` option for specifying
environment variables, which may be more convenient in some cases:
mach mochitest --setenv=XPCOM_MEM_LOG_CLASSES=MyClass [options]
For getting allocation stacks in automation, you can add the appropriate
`--setenv` options to the test configurations for the platforms you're
interested in. Those configurations are located in
`testing/mozharness/configs/`. The most likely configs you'll want to
modify are listed below:
- Linux: `unittests/linux_unittest.py`
- Mac: `unittests/mac_unittest.py`
- Windows: `unittests/win_unittest.py`
- Android: `android/androidarm.py`
## How to instrument your objects for BloatView
First, if your object is an xpcom object and you use the
`NS_IMPL_ADDREF` and `NS_IMPL_RELEASE` (or a variation thereof) macro to
implement your `AddRef` and `Release` methods, then there is nothing you
need do. By default, those macros support refcnt logging directly.
If your object is not an xpcom object then some manual editing is in
order. The following sample code shows what must be done:
MyType::MyType()
{
MOZ_COUNT_CTOR(MyType);
...
}
MyType::~MyType()
{
MOZ_COUNT_DTOR(MyType);
...
}
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