util.chunkbuffer: avoid extra mutations when reading partial chunks...
util.chunkbuffer: avoid extra mutations when reading partial chunks
Previously, a read(N) where N was less than the length of the first
available chunk would mutate the deque instance twice and allocate a new
str from the slice of the existing chunk. Profiling drawed my attention
to these as a potential hot spot during changegroup reading.
This patch makes the code more complicated in order to avoid the
aforementioned 3 operations.
On a pre-generated mozilla-central gzip bundle, this series has the
following impact on `hg unbundle` performance on my MacBook Pro:
before: 358.21 real 317.69 user 38.49 sys
after: 301.57 real 262.69 user 37.11 sys
delta: -56.64 real -55.00 user -1.38 sys
