streamclone: use backgroundfilecloser (issue4889)
Closing files that have been appended to is slow on Windows/NTFS.
CloseHandle() calls on this platform often take 1-10ms - and that's
on my i7-6700K Skylake processor with a modern and fast SSD. Contrast
with other I/O operations, such as writing data, which take <100us.
This means that creating/appending thousands of files can add
significant overhead. For example, cloning mozilla-central creates
~232,000 revlog files. Assuming 1ms per CloseHandle(), that yields
232s (3:52) of wall time waiting for file closes!
The impact of this overhead can be measured most directly when applying
stream clone bundles. Applying these files is effectively uncompressing
a tar archive (read: it's very fast).
Using a RAM disk (read: no I/O wait), the difference in wall time for a
`hg debugapplystreamclonebundle` for a ~1731 MB mozilla-central bundle
between Windows and Linux from the same machine is drastic:
Linux: ~12.8s (128MB/s)
Windows: ~352.0s (4.7MB/s)
Windows is ~27.5x slower. Yikes!
After this patch:
Linux: ~12.8s (128MB/s)
Windows: ~102.1s (16.1MB/s)
Windows is now ~3.4x faster. Unfortunately, it is still ~8x slower than
Linux. Profiling reveals a few hot code paths that could likely be
improved. But those are for other patches.
This patch introduces test-clone-uncompressed.t because existing tests
of `clone --uncompressed` are scattered about and adding a variation for
background thread closing to e.g. test-http.t doesn't feel correct.
#require serve
$ hg init server
$ cd server
$ touch foo
$ hg -q commit -A -m initial
>>> for i in range(1024):
... with open(str(i), 'wb') as fh:
... fh.write(str(i))
$ hg -q commit -A -m 'add a lot of files'
$ hg serve -p $HGPORT -d --pid-file=hg.pid
$ cat hg.pid >> $DAEMON_PIDS
$ cd ..
Basic clone
$ hg clone --uncompressed -U http://localhost:$HGPORT clone1
streaming all changes
1027 files to transfer, 96.3 KB of data
transferred 96.3 KB in * seconds (*/sec) (glob)
searching for changes
no changes found
Clone with background file closing enabled
$ hg --debug --config worker.backgroundclose=true --config worker.backgroundcloseminfilecount=1 clone --uncompressed -U http://localhost:$HGPORT clone-background | grep -v adding
using http://localhost:$HGPORT/
sending capabilities command
sending branchmap command
streaming all changes
sending stream_out command
1027 files to transfer, 96.3 KB of data
starting 4 threads for background file closing
transferred 96.3 KB in * seconds (*/sec) (glob)
query 1; heads
sending batch command
searching for changes
all remote heads known locally
no changes found
sending getbundle command
bundle2-input-bundle: with-transaction
bundle2-input-part: "listkeys" (params: 1 mandatory) supported
bundle2-input-part: "listkeys" (params: 1 mandatory) supported
bundle2-input-bundle: 1 parts total
checking for updated bookmarks
preparing listkeys for "phases"
sending listkeys command
received listkey for "phases": 58 bytes