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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>