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.

mercurial/configitems.py

@@ -417,6 +417,9 b" 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,
 )

mercurial/localrepo.py

@@ -611,8 +611,11 b' 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-'):

mercurial/revlog.py

@@ -180,7 +180,7 b' 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 b' 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 b' 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 '