fix(model): consider number of CPU cores when calculating hashers on interactive OS (#10284) (#10286)

Currently, the number of hashers is always set to 1 on interactive
operating systems, which are defined as Windows, macOS, iOS, and
Android. However, with modern multicore CPUs, it does not make much
sense to limit performance so much.

For example, without this fix, a CPU with 16 cores / 32 threads is
still limited to using just a single thread to hash files per folder,
which may severely affect its performance.

For this reason, instead of using a fixed value, calculate the number
dynamically, so that it equals one-fourth of the total number of CPU
cores. This way, the value of hashes will still end up being just 1 on
a slower 4-thread CPU, but it will be allowed to take larger values when
the number of threads is higher, increasing hashing performance in the
process.

Signed-off-by: Tomasz Wilczyński <twilczynski@naver.com>
Co-authored-by: Jakob Borg <jakob@kastelo.net>

lib/model/model.go

@@ -2561,16 +2561,17 @@ func (m *model) numHashers(folder string) int {
 		return folderCfg.Hashers
 	}
 
+	numCpus := runtime.GOMAXPROCS(-1)
 	if build.IsWindows || build.IsDarwin || build.IsIOS || build.IsAndroid {
 		// Interactive operating systems; don't load the system too heavily by
 		// default.
-		return 1
+		numCpus = max(1, numCpus/4)
 	}
 
 	// For other operating systems and architectures, lets try to get some
 	// work done... Divide the available CPU cores among the configured
 	// folders.
-	if perFolder := runtime.GOMAXPROCS(-1) / numFolders; perFolder > 0 {
+	if perFolder := numCpus / numFolders; perFolder > 0 {
 		return perFolder
 	}