如果一个在代码仓库中发现了一个bug，如果能够将其定位到是某一次提交（commit）引起的，然后再来fix这个bug就会比较容易了。基本上可以说，定位到具体的问题点，就可以算解决了一半问题了。
在GIT、Mercurial(hg)等SCM工具中，提供了bisect命令用于二分查找(binary section search），只需要通过命令告诉GIT或Mercurial工具就可自动帮我们checkout到对应的中间版本之下，然后通过自己试验，循环地运行bisect命令，即可较快地找到哪个commit引入的bug（或者fix了某个bug）。
今天刚好就通过Mercurial (hg)的 “hg bisect”命令查找到了一个commit，它修复Xen-unstable tree里面的nested virtualization中L2 guest的AVX指令不能正常执行的bug。 BTW，这个changeset是：changeset 25789:825f78461622
可以通过 “man hg”命令，然后找到bisect命令的解释，使用起来非常方便的。
下面是使用hg bisect命令进行二分查找的过程示例：
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# better to have a reset to make the repo clean [root@vt-snb7 xen-unstable.hg]# hg bisect -r [root@vt-snb7 xen-unstable.hg]# hg bisect -b 25765 [root@vt-snb7 xen-unstable.hg]# hg bisect -g 26319 Testing changeset 26042:3696dd6a7836 (554 changesets remaining, ~9 tests) 462 files updated, 0 files merged, 39 files removed, 0 files unresolved # just have a double check 'hg bisect' did the right thing. # 'do not' use 'hg log' for this usage [root@vt-snb7 xen-unstable.hg]# hg parent changeset: 26042:3696dd6a7836 user: Ian Jackson <ian.jackson@eu.citrix.com> date: Tue Oct 09 18:00:00 2012 +0100 summary: docs, build: Do not ignore install-docs errors [root@vt-snb7 xen-unstable.hg]# hg bisect -g Testing changeset 25903:5e4a00b4114c (277 changesets remaining, ~8 tests) 239 files updated, 0 files merged, 23 files removed, 0 files unresolved [root@vt-snb7 xen-unstable.hg]# hg bisect -b Testing changeset 25972:16aaeaa66ef9 (139 changesets remaining, ~7 tests) 175 files updated, 0 files merged, 3 files removed, 0 files unresolved [root@vt-snb7 xen-unstable.hg]# hg bisect -g Testing changeset 25937:32187301ecc5 (69 changesets remaining, ~6 tests) 66 files updated, 0 files merged, 6 files removed, 0 files unresolved [root@vt-snb7 xen-unstable.hg]# hg bisect -b Testing changeset 25954:8f8fabafec86 (35 changesets remaining, ~5 tests) 43 files updated, 0 files merged, 0 files removed, 0 files unresolved [root@vt-snb7 xen-unstable.hg]# hg bisect -g Testing changeset 25945:0604d4676c6b (17 changesets remaining, ~4 tests) 17 files updated, 0 files merged, 2 files removed, 0 files unresolved [root@vt-snb7 xen-unstable.hg]# hg bisect -g Testing changeset 25941:795c493fe561 (8 changesets remaining, ~3 tests) 6 files updated, 0 files merged, 1 files removed, 0 files unresolved [root@vt-snb7 xen-unstable.hg]# hg bisect -g Testing changeset 25939:b49f7bf52fa9 (4 changesets remaining, ~2 tests) 4 files updated, 0 files merged, 0 files removed, 0 files unresolved [root@vt-snb7 xen-unstable.hg]# hg bisect -g Testing changeset 25938:8f658b463b78 (2 changesets remaining, ~1 tests) 4 files updated, 0 files merged, 0 files removed, 0 files unresolved [root@vt-snb7 xen-unstable.hg]# hg bisect -g The first good revision is: changeset: 25938:8f658b463b78 user: Jan Beulich <jbeulich@suse.com> date: Fri Sep 21 17:02:46 2012 +0200 summary: x86: enable VIA CPU support |