`pprof` and Remote Servers

In mid-2006, we added an experimental facility to pprof, the tool that analyzes CPU and heap profiles. This facility allows you to collect profile information from running applications. It makes it easy to collect profile information without having to stop the program first, and without having to log into the machine where the application is running. This is meant to be used on webservers, but will work on any application that can be modified to accept TCP connections on a port of its choosing, and to respond to HTTP requests on that port.

We do not currently have infrastructure, such as apache modules, that you can pop into a webserver or other application to get the necessary functionality "for free." However, it's easy to generate the necessary data, which should allow the interested developer to add the necessary support into his or her applications.

To use pprof in this experimental "server" mode, you give the script a host and port it should query, replacing the normal commandline arguments of application + profile file:

   % pprof internalweb.mycompany.com:80

The host must be listening on that port, and be able to accept HTTP/1.0 requests -- sent via wget and curl -- for several urls. The following sections list the urls that pprof can send, and the responses it expects in return.

/pprof/heap

pprof asks for the url /pprof/heap to get heap information. The actual url is controlled via the variable HEAP_PAGE in the pprof script, so you can change it if you'd like.

The server should respond by calling

    MallocExtension::instance()->GetHeapSample(&output);

and sending output back as an HTTP response to pprof. MallocExtension is defined in the header file google/malloc_extension.h.

Here's an example, from an actual Google webserver, of what the output should look like:

heap profile:   9369: 126987529 [  9369: 126987529] @ heap
     2:     1024 [     2:     1024] @ 0x87da913 0x8923ad4 0x891d4c2 0x892de12 0x8930519 0x83a16c2 0x836cb38 0x834cd1c 0x8349ba5 0x10a3177 0x8349961
     1:       36 [     1:       36] @ 0x87da913 0x83a0929 0x836cb38 0x834cd1c 0x8349ba5 0x10a3177 0x8349961
   308: 10092544 [   308: 10092544] @ 0x87da913 0x8970d66 0x8970e64 0x896e8e2 0x88e69d2 0x88e6add 0x88e6dec 0x88e7384 0x88e73fa 0x8838793 0x8838b36 0x88395f8 0x88f5a4b 0x890d03a 0x890d65a 0x8917666 0x890d1f3 0x890e6e4 0x8349c1b 0x10a3177 0x8349961
[...]

/pprof/growth
pprof asks for the url /pprof/growth to get heap-profiling delta (growth) information. The actual url is controlled via the variable GROWTH_PAGE in the pprof script, so you can change it if you'd like.

The server should respond by calling
```
    MallocExtension::instance()->GetHeapGrowthStacks(&output);
```
and sending output back as an HTTP response to pprof. MallocExtension is defined in the header file google/malloc_extension.h.

Here's an example, from an actual Google webserver, of what the output should look like:
```
heap profile:    741: 812122112 [   741: 812122112] @ growth
     1:  1572864 [     1:  1572864] @ 0x87da564 0x87db8a3 0x84787a4 0x846e851 0x836d12f 0x834cd1c 0x8349ba5 0x10a3177 0x8349961
     1:  1048576 [     1:  1048576] @ 0x87d92e8 0x87d9213 0x87d9178 0x87d94d3 0x87da9da 0x8a364ff 0x8a437e7 0x8ab7d23 0x8ab7da9 0x8ac7454 0x8348465 0x10a3161 0x8349961
[...]
```
/pprof/profile
pprof asks for the url /pprof/profile?seconds=XX to get cpu-profiling information. The actual url is controlled via the variable PROFILE_PAGE in the pprof script, so you can change it if you'd like.

The server should respond by calling ProfilerStart(filename), continuing to do its work, and then, XX seconds later, calling ProfilerStop(). (These functions are declared in google/profiler.h.) The application is responsible for picking a unique filename for ProfilerStart(). After calling ProfilerStop(), the server should read the contents of filename and send them back as an HTTP response to pprof.

Obviously, to get useful profile information the application must continue to run in the XX seconds that the profiler is running. Thus, the profile start-stop calls should be done in a separate thread, or be otherwise non-blocking.

The profiler output file is binary, but near the end of it, it should have lines of text somewhat like this:
```
01016000-01017000 rw-p 00015000 03:01 59314      /lib/ld-2.2.2.so
```
/pprof/contention
This is intended to be able to profile (thread) lock contention in addition to CPU and memory use. It's not yet usable.
/pprof/cmdline
pprof asks for the url /pprof/cmdline to figure out what application it's profiling. The actual url is controlled via the variable PROGRAM_NAME_PAGE in the pprof script, so you can change it if you'd like.

The server should respond by reading the contents of /proc/self/cmdline, converting all internal NUL (\0) characters to newlines, and sending the result back as an HTTP response to pprof.

Here's an example return value:
```
/root/server/custom_webserver
80
--configfile=/root/server/ws.config
```
/pprof/symbol
pprof asks for the url /pprof/symbol to map from hex addresses to variable names. The actual url is controlled via the variable SYMBOL_PAGE in the pprof script, so you can change it if you'd like.

This is perhaps the hardest request to write code for, because it must accept POST requests. This means that after the HTTP headers, pprof will pass in a list of hex addresses connected by +, like so:
```
   curl -d '0x0824d061+0x0824d1cf' http://remote_host:80/pprof/symbol
```
The server should read the POST data, which will be in one line, and for each hex value, should write one line of output to the output stream, like so:
```
<hex address><tab><function name>
```
For instance:
```
0x08b2dabd    _Update
```
The other reason this is the most difficult request to implement, is that the application will have to figure out for itself how to map from address to function name. One possibility is to run nm -C -n <program name> to get the mappings, either statically (say at program-compile time), or dynamically, by having the application call out to nm for every pprof/symbol call (presumably with some caching!).

pprof itself does just this for local profiles (not ones that talk to remote servers); look at the subroutine GetProcedureBoundaries.

Last modified: Mon Jun 12 21:30:14 PDT 2006

pprof and Remote Servers

`pprof` and Remote Servers