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−FIXME+ - write about type of trees+ - use of kcmp syscall+ - internal representation
+
== Overview ==
−Usually we dump not just a single process but a set of them, where every process may be sharing some resources with other processes.
−Thus we need somehow to distinguish which resources are shared and which are not.
−For this sake [http://man7.org/linux/man-pages/man2/kcmp.2.html kcmp] system call has been introduced to Linux kernel.
+It takes two processes and compare a resource asked, returning result similar to well known
+[http://man7.org/linux/man-pages/man3/strcmp.3.html strcmp] call. This allows CRIU to track resources with a sorting algorithm.
+
+=== Kcmp-trees ===
+
+CRIU gather files, filesystems, vitrual memory descriptors, signal handlers and file descriptors associated with a process
+each into Kcmp-tree. Thus at moment we are carrying five Kcmp-trees. Each declared with <code>DECLARE_KCMP_TREE</code> helper.
+For example
+
+<pre>DECLARE_KCMP_TREE(vm_tree, KCMP_VM);</pre>
+
+Each tree internally represented as [http://en.wikipedia.org/wiki/Red%E2%80%93black_tree red-black] tree.
+
+=== Requesting new ID ===
+
+When CRIU gathers process resources it check if a resource is already sitting inside of a tree calling
+<code>kid_generate_gen()</code> helper. If a resource is not in a tree - it pushed into a tree
+and a caller obtains new abstract ID which may be used inside CRIU images, otherwise the helper
+returns zero notifying that this kind of resource already known to CRIU and has been handled earlier.
+
+This feature is quite important to eliminate duplication of entries inside CRIU dump images, because
+two processes might share a lot of resources and dumping them multiple times would cause very serious
+performance issue.
[[Category: Under the hood]]
[[Category: Under the hood]]