gofmt is pretty heavily CPU-bound, since parsing and formatting 1MiB
of Go code takes much longer than reading that amount of bytes from
disk. However, parsing and manipulating a large Go source file is very
difficult to parallelize, so we continue to process each file in its
own goroutine.

A Go module may contain a large number of Go source files, so we need
to bound the amount of work in flight. However, because the
distribution of sizes for Go source files varies widely — from tiny
doc.go files containing a single package comment all the way up to
massive API wrappers generated by automated tools — the amount of
time, work, and memory overhead needed to process each file also
varies. To account for this variability, we limit the in-flight work
by bytes of input rather than by number of files. That allows us to
make progress on many small files while we wait for work on a handful
of large files to complete.

The gofmt tool has a well-defined output format on stdout, which was
previously deterministic. We keep it deterministic by printing the
results of each file in order, using a lazily-synchronized io.Writer
(loosly inspired by Haskell's IO monad). After a file has been
formatted in memory, we keep it in memory (again, limited by the
corresponding number of input bytes) until the output for all previous
files has been flushed. This adds a bit of latency compared to
emitting the output in nondeterministic order, but a little extra
latency seems worth the cost to preserve output stability.

This change is based on Daniel Martí's work in CL 284139, but using a
weighted semaphore and ephemeral goroutines instead of a worker pool
and batches. Benchmark results are similar, and I find the concurrency
in this approach a bit easier to reason about.

In the batching-based approach, the batch size allows us to "look
ahead" to find large files and start processing them early. To keep
the CPUs saturated and prevent stragglers, we would need to tune the
batch size to be about the same as the largest input files. If the
batch size is set too high, a large batch of small files could turn
into a straggler, but if the batch size is set too low, the largest
files in the data set won't be started early enough and we'll end up
with a large-file straggler.

One possible alternative would be to sort by file size instead of
batching: identify all of the files to be processed, sort from largest
to smallest, and then process the largest files first so that the
"tail" of processing covers the smallest files. However, that approach
would still fail to saturate available CPU when disk latency is high,
would require buffering an arbitrary amount of metadata in order to
sort by size, and (perhaps most importantly!) would not allow the
`gofmt` binary to preserve the same (deterministic) output order that
it has today.

In contrast, with a semaphore we can produce the same deterministic
output as ever using only one tuning parameter: the memory footprint,
expressed as a rough lower bound on the amount of RAM available per
thread. While we're below the memory limit, we can run arbitrarily
many disk operations arbitrarily far ahead, and process the results of
those operations whenever they become avaliable. Then it's up to the
kernel (not us) to schedule the disk operations for throughput and
latency, and it's up to the runtime (not us) to schedule the
goroutines so that they complete quickly.

In practice, even a modest assumption of a few megabytes per thread
seems to provide a nice speedup, and it should scale reasonably even
to machines with vastly different ratios of CPU to disk. (In practice,
I expect that most 'gofmt' invocations will work with files on at most
one physical disk, so the CPU:disk ratio should vary more-or-less
directly with the thread count, whereas the CPU:memory ratio is
more-or-less independent of thread count.)

name \ time/op         baseline.txt  284139.txt    simplified.txt
GofmtGorootCmd           11.9s ± 2%     2.7s ± 3%       2.8s ± 5%

name \ user-time/op    baseline.txt  284139.txt    simplified.txt
GofmtGorootCmd           13.5s ± 2%    14.4s ± 1%      14.7s ± 1%

name \ sys-time/op     baseline.txt  284139.txt    simplified.txt
GofmtGorootCmd           465ms ± 8%    229ms ±28%      232ms ±31%

name \ peak-RSS-bytes  baseline.txt  284139.txt    simplified.txt
GofmtGorootCmd          77.7MB ± 4%  162.2MB ±10%    192.9MB ±15%

For #43566

Change-Id: I4ba251eb4d188a3bd1901039086be57f0b341910
Reviewed-on: https://go-review.googlesource.com/c/go/+/317975
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>