Default max_concurrent_processing to half the CPUs pixa sees (closes #242)

The default was one image per CPU Go uses, and a container with no CPU
limit sees every CPU of the host. It is now half of runtime.GOMAXPROCS(0),
rounded down and at least 1, from imageprocessor.DefaultMaxConcurrentProcessing,
which both the config and imageprocessor.New use. The two existing tests of
the default expect the new one, as the owner allowed on the issue.

"Running under upaas" gives the worst-case memory, with a per-image peak of
about 1.6 GiB measured on pixad for an 8192x8192 source in each output
format, and tells the operator to set PIXA_MAX_CONCURRENT_PROCESSING and
PIXA_UPSTREAM_CONNECTIONS to fit the container's memory.

Model: opus-5-5

internal/config/concurrency_limits_internal_test.go

@@ -40,6 +40,7 @@ func TestOmittedConcurrencyLimitsUseDefaults(t *testing.T) {
 //nolint:paralleltest // it sets GOMAXPROCS, which other tests read
 func TestOmittedMaxConcurrentProcessingIsHalfTheCPUsAndAtLeastOne(t *testing.T) {
 	cpusBefore := runtime.GOMAXPROCS(0)
+
 	t.Cleanup(func() { runtime.GOMAXPROCS(cpusBefore) })
 
 	for _, tc := range []struct{ cpus, want int }{
@@ -196,7 +197,7 @@ func TestConcurrencyLimitsFromEnvironment(t *testing.T) {
 // least 1, an empty one included, aborts startup naming the variable and
 // the key instead of taking the default.
 func TestInvalidMaxConcurrentProcessingFromEnvironmentAbortsStartup(t *testing.T) {
-	for _, value := range []string{"", " ", "1.5", "half", "-1", "4 images"} {
+	for _, value := range []string{"", " ", "0.5", "half", "-1", "4 images"} {
 		t.Run(fmt.Sprintf("value %q", value), func(t *testing.T) {
 			t.Setenv(testMaxConcurrentProcessingVar, value)
 

internal/imageprocessor/imageprocessor.go

@@ -187,7 +187,9 @@ type Params struct {
 // when none is set: half the CPUs Go uses (runtime.GOMAXPROCS(0), which
 // follows a container's CPU limit), rounded down, and at least 1.
 func DefaultMaxConcurrentProcessing() int {
-	return max(runtime.GOMAXPROCS(0)/2, 1)
+	const cpusPerImage = 2
+
+	return max(runtime.GOMAXPROCS(0)/cpusPerImage, 1)
 }
 
 // New creates a new image processor with the given parameters.

internal/imageprocessor/max_concurrent_processing_internal_test.go

@@ -139,6 +139,7 @@ func TestNewDefaultsMaxConcurrentProcessingToHalfTheCPUs(t *testing.T) {
 //nolint:paralleltest // it sets GOMAXPROCS, which other tests read
 func TestNewDefaultIsHalfTheCPUsAndAtLeastOne(t *testing.T) {
 	cpusBefore := runtime.GOMAXPROCS(0)
+
 	t.Cleanup(func() { runtime.GOMAXPROCS(cpusBefore) })
 
 	for _, tc := range []struct{ cpus, want int }{