testwrapper ran a separate, sequential "go test" invocation for each
package pattern on its command line. That is fine for a single "./..."
argument but not for callers that pass an explicit package list: CI
jobs in the corp repo passing ~200 packages ran ~200 serial go test
processes with no cross-package parallelism and a fixed set of
never-cacheable lookups per process, and spent several times longer
on process startup, package loading, cache lookups, and serial test
binary links than on running tests. See tailscale/corp#48453 for the
details.
Locally, on 203 packages with a fully warm build and test cache, so
measuring only the per-invocation overhead:
old (203 go test processes): 26.4s
new (1 go test process): 3.6s (7.3x faster)
Our own Windows CI job hits the same path: its "sharded:N/M" mode
expands to an explicit list of that shard's packages via listpkgs, so
each shard ran one go test process per package, and Windows process
startup is slower still. Each shard now runs as one invocation.
Fixestailscale/corp#48453
Updates tailscale/corp#47035
Change-Id: I3f796ff1724af40f93be9f918a7ddfde3bb45a91
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Brad Fitzpatrick
1033e714ca