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1 | <html> | ||
2 | <head> | ||
3 | <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> | ||
4 | <title>3.2. Shared State Cache</title> | ||
5 | <link rel="stylesheet" type="text/css" href="../book.css"> | ||
6 | <meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> | ||
7 | <link rel="home" href="index.html" title="The Yocto Project Reference Manual"> | ||
8 | <link rel="up" href="technical-details.html" title="Chapter 3. Technical Details"> | ||
9 | <link rel="prev" href="usingpoky-components-configuration.html" title="3.1.4. Configuration"> | ||
10 | <link rel="next" href="overall-architecture.html" title="3.2.1. Overall Architecture"> | ||
11 | </head> | ||
12 | <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="section" title="3.2. Shared State Cache"> | ||
13 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | ||
14 | <a name="shared-state-cache"></a>3.2. Shared State Cache</h2></div></div></div> | ||
15 | <p> | ||
16 | By design, the OpenEmbedded build system builds everything from scratch unless | ||
17 | BitBake can determine that parts don't need to be rebuilt. | ||
18 | Fundamentally, building from scratch is attractive as it means all parts are | ||
19 | built fresh and there is no possibility of stale data causing problems. | ||
20 | When developers hit problems, they typically default back to building from scratch | ||
21 | so they know the state of things from the start. | ||
22 | </p> | ||
23 | <p> | ||
24 | Building an image from scratch is both an advantage and a disadvantage to the process. | ||
25 | As mentioned in the previous paragraph, building from scratch ensures that | ||
26 | everything is current and starts from a known state. | ||
27 | However, building from scratch also takes much longer as it generally means | ||
28 | rebuilding things that don't necessarily need rebuilt. | ||
29 | </p> | ||
30 | <p> | ||
31 | The Yocto Project implements shared state code that supports incremental builds. | ||
32 | The implementation of the shared state code answers the following questions that | ||
33 | were fundamental roadblocks within the OpenEmbedded incremental build support system: | ||
34 | </p> | ||
35 | <div class="itemizedlist"><ul class="itemizedlist" type="disc"> | ||
36 | <li class="listitem">What pieces of the system have changed and what pieces have not changed?</li> | ||
37 | <li class="listitem">How are changed pieces of software removed and replaced?</li> | ||
38 | <li class="listitem">How are pre-built components that don't need to be rebuilt from scratch | ||
39 | used when they are available?</li> | ||
40 | </ul></div> | ||
41 | <p> | ||
42 | </p> | ||
43 | <p> | ||
44 | For the first question, the build system detects changes in the "inputs" to a given task by | ||
45 | creating a checksum (or signature) of the task's inputs. | ||
46 | If the checksum changes, the system assumes the inputs have changed and the task needs to be | ||
47 | rerun. | ||
48 | For the second question, the shared state (sstate) code tracks which tasks add which output | ||
49 | to the build process. | ||
50 | This means the output from a given task can be removed, upgraded or otherwise manipulated. | ||
51 | The third question is partly addressed by the solution for the second question | ||
52 | assuming the build system can fetch the sstate objects from remote locations and | ||
53 | install them if they are deemed to be valid. | ||
54 | </p> | ||
55 | <p> | ||
56 | The rest of this section goes into detail about the overall incremental build | ||
57 | architecture, the checksums (signatures), shared state, and some tips and tricks. | ||
58 | </p> | ||
59 | </div></body> | ||
60 | </html> | ||