comparison mercurial/streamclone.py @ 27897:2fdbf22a1b63

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.

27896:1d29893240cc

        handled_bytes = 0
        repo.ui.progress(_('clone'), 0, total=bytecount)
        start = time.time()

        with repo.transaction('clone'):
            if True:
                for i in xrange(filecount):
                    # XXX doesn't support '\n' or '\r' in filenames
                    l = fp.readline()
                    try:
                        name, size = l.split('\0', 1)

                            _('unexpected response from remote server:'), l)
                    if repo.ui.debugflag:
                        repo.ui.debug('adding %s (%s)\n' %
                                      (name, util.bytecount(size)))
                    # for backwards compat, name was partially encoded
                    with repo.svfs(store.decodedir(name), 'w') as ofp:
                        for chunk in util.filechunkiter(fp, limit=size):
                            handled_bytes += len(chunk)
                            repo.ui.progress(_('clone'), handled_bytes,
                                             total=bytecount)
                            ofp.write(chunk)

27897:2fdbf22a1b63

        handled_bytes = 0
        repo.ui.progress(_('clone'), 0, total=bytecount)
        start = time.time()

        with repo.transaction('clone'):
            with repo.svfs.backgroundclosing(repo.ui, expectedcount=filecount):
                for i in xrange(filecount):
                    # XXX doesn't support '\n' or '\r' in filenames
                    l = fp.readline()
                    try:
                        name, size = l.split('\0', 1)

                            _('unexpected response from remote server:'), l)
                    if repo.ui.debugflag:
                        repo.ui.debug('adding %s (%s)\n' %
                                      (name, util.bytecount(size)))
                    # for backwards compat, name was partially encoded
                    path = store.decodedir(name)
                    with repo.svfs(path, 'w', backgroundclose=True) as ofp:
                        for chunk in util.filechunkiter(fp, limit=size):
                            handled_bytes += len(chunk)
                            repo.ui.progress(_('clone'), handled_bytes,
                                             total=bytecount)
                            ofp.write(chunk)