21639: Note about 0 prefetch threads

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>

diff --git a/services/fuse/arvados_fuse/command.py b/services/fuse/arvados_fuse/command.py
index 29ace2e52e6f82b1b784049cd8fc05baeac75575..4c8ae899e12834e0b17ebbfbcdf27dc3907a46ce 100644 (file)
--- a/services/fuse/arvados_fuse/command.py
+++ b/services/fuse/arvados_fuse/command.py
@@ -482,12 +482,11 @@ class Mount(object):
                                                       disk_cache=self.args.disk_cache,
                                                       disk_cache_dir=self.args.disk_cache_dir)
 
-            # If there's too many prefetch threads and you
-            # max out the CPU, delivering data to the FUSE
-            # layer actually ends up being slower.
-            # Experimentally, capping 7 threads seems to
-            # be a sweet spot.
-            #prefetch_threads = min(max((block_cache.cache_max // (64 * 1024 * 1024)) - 1, 1), 7)
+            # Profiling indicates that prefetching has more of a
+            # negative impact on the read() fast path (by requiring it
+            # to do more work and take additional locks) than benefit.
+            # Also, the kernel does some readahead itself, which has a
+            # similar effect.
             prefetch_threads = 0
 
             self.api = arvados.safeapi.ThreadSafeApiCache(