Comments
Patch
@@ -152,11 +152,33 @@ if os.name != 'nt':
_exitstatus = _posixexitstatus
def partition(lst, nslices):
'''partition a list into N slices of roughly equal size
The current strategy takes every Nth element from the input. If
we ever write workers that need to preserve grouping in input
we should consider allowing callers to specify a partition strategy.
+
+ mpm is not a fan of this partitioning strategy when files are involved.
+ In his words:
+
+ Single-threaded Mercurial makes a point of creating and visiting
+ files in a fixed order (alphabetical). When creating files in order,
+ a typical filesystem is likely to allocate them on nearby regions on
+ disk. Thus, when revisiting in the same order, locality is maximized
+ and various forms of OS and disk-level caching and read-ahead get a
+ chance to work.
+
+ This effect can be quite significant on spinning disks. I discovered it
+ circa Mercurial v0.4 when revlogs were named by hashes of filenames.
+ Tarring a repo and copying it to another disk effectively randomized
+ the revlog ordering on disk by sorting the revlogs by hash and suddenly
+ performance of my kernel checkout benchmark dropped by ~10x because the
+ "working set" of sectors visited no longer fit in the drive's cache and
+ the workload switched from streaming to random I/O.
+
+ What we should really be doing is have workers read filenames from a
+ ordered queue. This preserves locality and also keeps any worker from
+ getting more than one file out of balance.
'''
for i in range(nslices):
yield lst[i::nslices]