bees: soft-limit computed thread counts to 8

https://github.com/Zygo/bees/issues/91 describes problems encountered
when running bees on systems with many CPU cores.

Limit the computed number of threads (using --thread-factor or the
default) to a maximum of 8 (i.e. the number of logical cores in a modern
laptop).  Users can override the limit by using --thread-count.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

README.md

@@ -336,6 +336,12 @@ Unfixed kernel bugs (as of 4.14.34) with workarounds in Bees:
   or prealloc.  Bees avoids feedback loops this can generate while
   attempting to replace extents over 16MB in length.
 
+* **Systems with many CPU cores** may [lock up when bees runs with one
+  worker thread for every core](https://github.com/Zygo/bees/issues/91).
+  bees limits the number of threads it will try to create based on
+  detected CPU core count.  Users may override this limit with the
+  [`--thread-count` option](options.md).
+
 Not really bugs, but gotchas nonetheless:
 
 * If a process holds a directory FD open, the subvol containing the
@@ -536,12 +542,18 @@ Command Line Options
 
 * --thread-count (-c) COUNT
   * Specify maximum number of worker threads for scanning.  Overrides
-    --thread-factor (-C) and default/autodetected values.
+    --thread-factor (-C) and default/autodetected values,
+    and the hardcoded thread limit.
 * --thread-factor (-C) FACTOR
   * Specify ratio of worker threads to CPU cores.  Overridden by --thread-count (-c).
     Default is 1.0, i.e. 1 worker thread per detected CPU.  Use values
     below 1.0 to leave some cores idle, or above 1.0 if there are more
     disks than CPUs in the filesystem.
+    If the computed thread count is higher than `BEES_DEFAULT_THREAD_LIMIT`
+    (currently 8), then only that number of threads will be created.
+    This limit can be overridden by the `--thread-count` option; however,
+    be aware that there are kernel issues with systems that have many CPU
+    cores when users try to run bees on all of them.
 * --loadavg-target (-g) LOADAVG
   * Specify load average target for dynamic worker threads.
     Threads will be started or stopped subject to the upper limit imposed

src/bees.cc

@@ -785,6 +785,11 @@ bees_main(int argc, char *argv[])
 			thread_factor = BEES_DEFAULT_THREAD_FACTOR;
 		}
 		thread_count = max(1U, static_cast<unsigned>(ceil(thread::hardware_concurrency() * thread_factor)));
+		if (thread_count > BEES_DEFAULT_THREAD_LIMIT) {
+			BEESLOGNOTICE("Limiting computed thread count to " << BEES_DEFAULT_THREAD_LIMIT);
+			BEESLOGNOTICE("Use --thread-count to override this limit");
+			thread_count = BEES_DEFAULT_THREAD_LIMIT;
+		}
 	}
 
 	if (load_target != 0) {

src/bees.h

@@ -85,6 +85,9 @@ const size_t BEES_OPEN_FILE_LIMIT = (BEES_FILE_FD_CACHE_SIZE + BEES_ROOT_FD_CACH
 // Worker thread factor (multiplied by detected number of CPU cores)
 const double BEES_DEFAULT_THREAD_FACTOR = 1.0;
 
+// Don't use more than this number of threads unless explicitly configured
+const size_t BEES_DEFAULT_THREAD_LIMIT = 8;
+
 // Log warnings when an operation takes too long
 const double BEES_TOO_LONG = 5.0;