create verbosemmap.py

  $ cat << EOF > verbosemmap.py

  > # extension to make util.mmapread verbose

  > 

  > from __future__ import absolute_import

  > 

  > from mercurial import (

  >     extensions,

  >     pycompat,

  >     util,

  > )

  > 

  > def extsetup(ui):

  >     def mmapread(orig, fp):

  >         ui.write(b"mmapping %s\n" % pycompat.bytestr(fp.name))

  >         ui.flush()

  >         return orig(fp)

  > 

  >     extensions.wrapfunction(util, 'mmapread', mmapread)

  > EOF

setting up base repo

  $ hg init a

  $ cd a

  $ touch a

  $ hg add a

  $ hg commit -qm base

  $ for i in `$TESTDIR/seq.py 1 100` ; do

  > echo $i > a

  > hg commit -qm $i

  > done

set up verbosemmap extension

  $ cat << EOF >> $HGRCPATH

  > [extensions]

  > verbosemmap=$TESTTMP/verbosemmap.py

  > EOF

mmap index which is now more than 4k long

  $ hg log -l 5 -T '{rev}\n' --config experimental.mmapindexthreshold=4k

  mmapping $TESTTMP/a/.hg/store/00changelog.i

  100

  99

  98

  97

  96

do not mmap index which is still less than 32k

  $ hg log -l 5 -T '{rev}\n' --config experimental.mmapindexthreshold=32k

  100

  99

  98

  97

  96

  $ cd ..