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<task id="GUID-85C18DAF-DB76-51C6-B38D-A802E314F4D1" xml:lang="en"><title>Performance
Guarantee Tutorial </title><shortdesc>Describes how to maintain performance guarantees when writing a
software package on a data paged platform. </shortdesc><prolog><metadata><keywords/></metadata></prolog><taskbody>
<context id="GUID-067EBD71-74FF-5C3C-BBFE-792895383980"><p>Data paging can
affect performance guarantees. When writing software for a data paged platform,
you need to perform an impact assessment on the components of your application.
To do so, you perform the following analyses and implement the appropriate
mitigation for the impacts you discover: </p> <ul>
<li id="GUID-902266C8-BC97-5C0A-9C61-13F252CE3DC8"><p>Static analysis, </p> </li>
<li id="GUID-70E2A254-1799-5A1E-A440-6E7F30EC3337"><p>Use case analysis, and </p> </li>
<li id="GUID-B5A5134E-CD51-59BA-9F05-0DFA2E200660"><p>IPC analysis. </p> </li>
</ul> </context>
<steps-unordered id="GUID-5AECEB9A-F5AC-5D72-A216-F31C8E024453">
<step id="GUID-B5C885F6-8703-5E43-8979-72A2D11007AB"><cmd>Perform a static
analysis of the impact of data paging on your code. </cmd>
<substeps id="GUID-31FA74DF-5D4D-5DE5-96D4-A1978187E40D">
<substep id="GUID-C9722F09-D534-5CC3-AC25-238C0C101DC7"><cmd/>
<info>List all the executables in your package, indicating their size and
pageability. </info>
</substep>
<substep id="GUID-6193E0A0-AFB1-5641-973F-0D27F721983F"><cmd/>
<info>For each item in the list identify its static dependencies, indicating
their size and pageability. </info>
</substep>
<substep id="GUID-509636D1-3B7C-546D-8EC4-A7AE515B904E"><cmd/>
<stepxmp>For each item in the list identify its dynamic dependencies, indicating
their size and pageability. </stepxmp>
<info>For instance, note whether the component is a plug-in to a framework
or is itself a framework. </info>
</substep>
</substeps>
<stepresult>The purpose of collecting this information is to identify which
executables are most likely to be impacted by data paging. </stepresult>
</step>
<step id="GUID-469BB5C7-9BC7-5E3B-9717-B5558D2F6859"><cmd>Perform a use case
analysis of the impact of data paging on your code. It is good practice to
distinguish between use cases involving real time and performance critical
performance guarantees. You do not need to cover use cases which do not involve
guarantees of this kind. </cmd>
<substeps id="GUID-CC4B31C6-85B3-53FA-AEB1-4713B0F7E51C">
<substep id="GUID-4251C5AC-9F23-5850-A9C3-E086B00E45D9"><cmd>List all the
use cases for the component which involve real time performance guarantees. </cmd>
<info>Real time use cases are those having very strict response time guarantees
and also those involving an acceptable user perception of behaviour. </info>
<info><ul>
<li id="GUID-F24C5888-773A-5F17-911F-1424D5381616"><p>Video playback, </p> </li>
<li id="GUID-2C9B272C-265A-5A82-B9A0-A0CB5D2CC165"><p>VoIP phone calls, and </p> </li>
<li id="GUID-20D52A34-F21E-565E-9698-8E0A169B7E3F"><p>File download over USB. </p> </li>
</ul> </info>
<stepresult>Examples of use cases involving acceptable user perception are: </stepresult>
</substep>
<substep id="GUID-702A77F3-ACE4-5047-825E-267747AF4E70"><cmd>List all the
use cases for the component which involve performance critical guarantees.
This means the bench-marked use cases by which the performance of the operating
system is measure. </cmd>
<stepxmp>Examples of use cases having a strict response time are: </stepxmp>
<info><ul>
<li id="GUID-C2CAE80F-F20F-5464-9A5D-77858D26572C"><p>Standard boot time, </p> </li>
<li id="GUID-379EF955-B1D2-5A4F-A9CA-295BCDA6D84C"><p>Application start-up
time, and </p> </li>
<li id="GUID-5111E2E5-9411-5BD6-BC60-B1E64A3DBE57"><p>Camera image capture
time. </p> </li>
</ul> </info>
</substep>
<substep id="GUID-C6089DA1-C2A8-5D57-8F1C-B5D94D01CD31"><cmd>Identify compound
cases involving both real time and performance critical guarantees. </cmd>
<info>An example of a compound case is receiving a text message while playing
MP3 audio. </info>
</substep>
</substeps>
</step>
<step id="GUID-13DE4D82-D318-5C1B-A9A0-9C8D60C7A6B7"><cmd>Perform an IPC analysis
of the impact of data paging on your code. </cmd>
<substeps id="GUID-C53DD02E-D670-5608-9293-663495544410">
<substep id="GUID-14395B5D-B0C1-5380-B6D4-A9A5D5FA5A16"><cmd>Identify all
performance critical or real time servers which the component interacts with. </cmd>
</substep>
<substep id="GUID-B4429AD3-5B5F-5E10-A220-724BA9693D29"><cmd>List each case
where these servers read paged data from a client's address space. In this
case, paged data includes: </cmd>
<info><ul>
<li id="GUID-02F9AD09-BFEB-59D3-98C6-471A2D842F32"><p>Paged heaps and stacks, </p> </li>
<li id="GUID-BEA351B7-27C8-54F4-A2A9-F0314153D1F8"><p>RAM-loaded code, and </p> </li>
<li id="GUID-AE1A2E25-C84A-5CCB-AC03-40017B1E5EEF"><p>Read only XIP data structures
including </p> <ul>
<li id="GUID-A61629A2-6782-52E6-A64B-41825F454B1B"><p>bitmaps, </p> </li>
<li id="GUID-B5657762-F2E1-5521-AB33-E95547BA154B"><p>constant descriptors, </p> </li>
<li id="GUID-0A667519-6532-52C4-BCCD-022A44F701D6"><p>constant data arrays
in code, </p> </li>
<li id="GUID-E8C94651-CAA0-5EFE-8E87-6B7A7B7A8785"><p>data files accessed
through a pointer, and </p> </li>
<li id="GUID-CD319825-B44F-5EF3-A9CC-2B3391F2D9A8"><p>exported DLL data. </p> </li>
</ul> </li>
</ul> </info>
</substep>
<substep id="GUID-CB118001-AC5E-5C39-81C1-EB9DEFA2F16E"><cmd>Identify all
cases of custom architectures where one thread reads from another thread's
address space. </cmd>
</substep>
</substeps>
</step>
<step id="GUID-009892B8-88B2-5ACA-BB98-B88ECF49A983"><cmd>Take steps to mitigate
the impacts of data paging which you have identified, as appropriate to each
case. </cmd>

<substeps id="GUID-63086536-F2DB-5F4A-926F-C0328FD8D854">
<substep id="GUID-F2821854-2A7D-5FB4-8D75-63841CDD26F7"><cmd>Protect the real
time and performance critical code paths. These paths will have been identified
in the static analysis of the component. There are two ways of protecting
them. </cmd>
<info><ul>
<li id="GUID-5CA87396-D31D-5AC2-B524-0405129AEC23"><p>For all use cases, ensure
that the minimum paging cache size is large enough to accommodate both the
protected code path and any other paged data required at the same time. </p> <p>This
method of protection may not always be practical, because compound use cases
may involve unpredictable amounts of data which exceed the size of the paging
cache. </p> </li>
<li id="GUID-A702633C-A6DB-5DB2-9FA0-20F8FF4A285E"><p>Make the protected code
path unpaged. Mark the code itself as unpaged, and either mark whole regions
of memory as unpaged or pin memory to ensure that it is unpaged. </p> </li>
</ul> </info>
</substep>
<substep id="GUID-9486786D-191D-597F-9BCE-09CB3A74A165"><cmd>Redesign the
component architecture to isolate areas where data paging impacts on performance.
Examples of this are: </cmd>
<info><ul>
<li id="GUID-76E2623B-9B76-5424-9C40-3BC027C449A2"><p>Separating the data
plane and the control plane, and </p> </li>
<li id="GUID-26906CC5-1D1B-5EED-A9F8-CBBE6F441369"><p>Splitting a monolithic
library into paged and unpaged parts. </p> </li>
</ul> </info>
</substep>
<substep id="GUID-438E7745-76B3-533D-891E-93B8EA53F342"><cmd>Use code paging
or XIP ROM paging to mitigate the impact of paging. </cmd>

<info>For instance, a monolithic library with static dependences may only
need to be partially loaded, and some dependencies may not need to be loaded
at all. </info>
</substep>
<substep id="GUID-DD1D6948-8F9A-57D6-9156-C9C2E377D2FD"><cmd/>
<info>Mitigate the impact of data paging on IPC using the techniques set out
in the document <xref href="GUID-E7C55048-5B7A-5BF2-B7F4-4D731659B88C-GENID-1-7-1-14-1-1-7-1-7-1-11-1-7-1-4-1-9-1.dita">Device
Driver Writing Technology Tutorial</xref>  </info>
</substep>
</substeps>
</step>
</steps-unordered>
</taskbody><related-links>
<link href="GUID-91C11D80-7A62-5135-8EE0-6EC5576D1118.dita"><linktext>Demand Paged
Device Drivers Writing                 Guide</linktext></link>
</related-links></task>