docs: add perf section to developer docs (James O'Beirne)
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.
for i in range(200):
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:
$ perf report -i /tmp/testtxmpod0y/node0/node-0-TestName-large-msgs.perf.data --stdio \
| c++filt \