# HG changeset patch
# User Paul Morelle <paul.morelle@octobus.net>
# Date 2017-10-14 15:05:41
# Node ID 4d5d5009bd7544ac0c2c5366bd04bc4228a5b148
# Parent  e2ad93bcc084b97c48f54c179365376edb702858

revlog-sparse-read: add a lower-threshold for read block size

The option experimental.sparse-read.min-block-size specifies the minimal size
of a deltachain span, under which it won't be split by _slicechunk.

diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -417,6 +417,9 @@ coreconfigitem('experimental', 'sparse-r
 coreconfigitem('experimental', 'sparse-read.density-threshold',
     default=0.25,
 )
+coreconfigitem('experimental', 'sparse-read.min-block-size',
+    default='256K',
+)
 coreconfigitem('experimental', 'treemanifest',
     default=False,
 )
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -611,8 +611,11 @@ class localrepository(object):
         withsparseread = self.ui.configbool('experimental', 'sparse-read')
         srdensitythres = float(self.ui.config('experimental',
                                               'sparse-read.density-threshold'))
+        srminblocksize = self.ui.configbytes('experimental',
+                                             'sparse-read.min-block-size')
         self.svfs.options['with-sparse-read'] = withsparseread
         self.svfs.options['sparse-read-density-threshold'] = srdensitythres
+        self.svfs.options['sparse-read-min-block-size'] = srminblocksize
 
         for r in self.requirements:
             if r.startswith('exp-compression-'):
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -180,7 +180,7 @@ def _slicechunk(revlog, revs):
         endbyte = start(revs[-1]) + length(revs[-1])
         deltachainspan = endbyte - startbyte
 
-        if len(revs) <= 1:
+        if deltachainspan <= revlog._srminblocksize or len(revs) <= 1:
             yield revs
             continue
 
@@ -360,6 +360,7 @@ class revlog(object):
         self._maxdeltachainspan = -1
         self._withsparseread = False
         self._srdensitythreshold = 0.25
+        self._srminblocksize = 262144
 
         mmapindexthreshold = None
         v = REVLOG_DEFAULT_VERSION
@@ -389,6 +390,8 @@ class revlog(object):
             self._withsparseread = bool(opts.get('with-sparse-read', False))
             if 'sparse-read-density-threshold' in opts:
                 self._srdensitythreshold = opts['sparse-read-density-threshold']
+            if 'sparse-read-min-block-size' in opts:
+                self._srminblocksize = opts['sparse-read-min-block-size']
 
         if self._chunkcachesize <= 0:
             raise RevlogError(_('revlog chunk cache size %r is not greater '