13782b8ba8 docs: add perf section to developer docs (James O'Beirne)
58180b5fd4 tests: add utility to easily profile node performance with perf (James O'Beirne)
Pull request description:
Adds a context manager to easily (and selectively) profile node performance during functional test execution using `perf`.
While writing some tests, I encountered some odd bitcoind slowness. I wrote up a utility (`TestNode.profile_with_perf`) that generates performance diagnostics for a node by running `perf` during the execution of a particular region of test code.
`perf` usage is detailed in the excellent (and sadly unmerged) https://github.com/bitcoin/bitcoin/pull/12649; all due props to @eklitzke.
### Example
```python
with node.profile_with_perf("large-msgs"):
for i in range(200):
node.p2p.send_message(some_large_msg)
node.p2p.sync_with_ping()
```
This generates a perf data file in the test node's datadir (`/tmp/testtxmpod0y/node0/node-0-TestName-large-msgs.perf.data`).
Running `perf report` generates nice output about where the node spent most of its time while running that part of the test:
```bash
$ perf report -i /tmp/testtxmpod0y/node0/node-0-TestName-large-msgs.perf.data --stdio \
| c++filt \
| less
# To display the perf.data header info, please use --header/--header-only options.
#
#
# Total Lost Samples: 0
#
# Samples: 135 of event 'cycles:pp'
# Event count (approx.): 1458205679493582
#
# Children Self Command Shared Object Symbol
# ........ ........ ............... ................... ........................................................................................................................................................................................................................................................................
#
70.14% 0.00% bitcoin-net bitcoind [.] CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)
|
---CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)
70.14% 0.00% bitcoin-net bitcoind [.] CNetMessage::readData(char const*, unsigned int)
|
---CNetMessage::readData(char const*, unsigned int)
CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)
35.52% 0.00% bitcoin-net bitcoind [.] std::vector<char, zero_after_free_allocator<char> >::_M_fill_insert(__gnu_cxx::__normal_iterator<char*, std::vector<char, zero_after_free_allocator<char> > >, unsigned long, char const&)
|
---std::vector<char, zero_after_free_allocator<char> >::_M_fill_insert(__gnu_cxx::__normal_iterator<char*, std::vector<char, zero_after_free_allocator<char> > >, unsigned long, char const&)
CNetMessage::readData(char const*, unsigned int)
CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)
...
```
Tree-SHA512: 9ac4ceaa88818d5eca00994e8e3c8ad42ae019550d6583972a0a4f7b0c4f61032e3d0c476b4ae58756bc5eb8f8015a19a7fc26c095bd588f31d49a37ed0c6b3e
Introduces `TestNode.profile_with_perf()` context manager which
samples node execution to produce profiling data.
Also introduces a test framework flag, `--perf`, which will run
perf on all nodes for the duration of a given test.
Jon Atack