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<td id="email-title-avatar" rowspan="2" style="font-size: 13px; text-align: left; vertical-align: top; width: 48px; padding-right: 8px"> <img class="avatar" src="cid:avatar_f8db8921231053c83d89044f1839e1c0" border="0" height="48" width="48" style="-ms-interpolation-mode: bicubic; border-radius: 3px" /> </td>
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<a href=" https://wiki.asterisk.org/wiki/display/~jcolp " style="color:#326ca6;text-decoration:none;; color: #326ca6; text-decoration: none">Joshua Colp</a> created a page:
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<td valign="top" style="font-size: 13px"> <h2 id="email-title-heading" style="font-size: 16px; line-height: 20px; min-height: 20px; margin: 0; padding: 0"> <a href="https://wiki.asterisk.org/wiki/display/AST/Media+Format+Rewrite" style="color: #326ca6; text-decoration: none"> <img class="icon" src="cid:page-icon" alt="" style="border: 0; padding: 0 5px 0 0; text-align: left; vertical-align: middle" /> <strong style="font-size:16px;line-height:20px;vertical-align:top;">Media Format Rewrite</strong> </a> </h2> </td>
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<h1 id="MediaFormatRewrite-History">History</h1>
<p style="font-size: 13px">In the old times media formats were represented using a bit field. This was fast but had a few limitations. We were limited on how many there were and they also did not include any attribute information. It was strictly a "this is ulaw". This was changed and ast_format was created, which is a structure that contains additional information. Additionally ast_format_cap was created to act as a container and another mechanism was added to allow logic to be registered which performed format negotiation. Everywhere throughout the codebase the code was changed to use this strategy but unfortunately this came at a cost.</p>
<h1 id="MediaFormatRewrite-ThePresent">The Present</h1>
<h3 id="MediaFormatRewrite-structast_format">struct ast_format</h3>
<p style="font-size: 13px">Media formats in Asterisk are now represented using a large (~320 byte) sized data structure. This is done because the data structure itself is not a reference counted object and thus carries no guarantee that associated information attached to it will be disposed of. The large size of the data structure is due to needing additional space for optional media format attributes.</p>
<h3 id="MediaFormatRewrite-ast_format_copy">ast_format_copy</h3>
<p style="font-size: 13px">Copying formats now requires copying this large amount of memory. While one would think this occurs infrequently in practice this can occur more than 5 times for a single frame passing through Asterisk.</p>
<h3 id="MediaFormatRewrite-ast_format_cmp">ast_format_cmp</h3>
<p style="font-size: 13px">Comparing formats is no longer cheap either. Each comparison requires doing a container lookup to see if any additional logic is registered to augment the comparison operation. As code within Asterisk needs to be aware of when formats change this can occur 4 or more times for a single frame passing through Asterisk.</p>
<h3 id="MediaFormatRewrite-Containerlookupusingast_formataskey">Container lookup using ast_format as key</h3>
<p style="font-size: 13px">Since comparing formats are no longer cheap using the ast_format as a container key is extremely expensive.</p>
<h3 id="MediaFormatRewrite-Assumptionsnolongertrue">Assumptions no longer true</h3>
<p style="font-size: 13px">There are assumptions throughout the tree that media format related operations are cheap when in reality they are anything but. An example would be reusable frames. Instead of setting the frames up with the information they require at initialization they are set each time the frame is returned.</p>
<h1 id="MediaFormatRewrite-TheFuture">The Future</h1>
<p style="font-size: 13px"> </p>
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<td class="left" valign="top" style="font-size: 13px; padding-top: 8px; max-width: 45%; text-align: left"> <span class="email-list-item"><a href="https://wiki.asterisk.org/wiki/display/AST/Media+Format+Rewrite" style="color: #326ca6; text-decoration: none">View Online</a> </span> <span class="email-list-divider" style="color: #505050; padding: 0 0.350em">·</span> <span class="email-list-item"><a href="https://wiki.asterisk.org/wiki/plugins/likes/like.action?contentId=27199677" style="color: #326ca6; text-decoration: none">Like</a> </span> <span class="email-list-divider" style="color: #505050; padding: 0 0.350em">·</span> <span class="email-list-item"><a href="https://wiki.asterisk.org/wiki/display/AST/Media+Format+Rewrite?showComments=true&showCommentArea=true#addcomment" style="color: #326ca6; text-decoration: none">Add Comment</a> </span> </td>
<td class="right" width="50%" valign="top" style="font-size: 13px; padding-top: 8px; text-align: right"> <span class="email-list-item"><a href="https://wiki.asterisk.org/wiki/users/removespacenotification.action?spaceKey=AST" style="color: #326ca6; text-decoration: none">Stop watching space</a> </span> <span class="email-list-divider" style="color: #505050; padding: 0 0.350em">·</span> <span class="email-list-item"><a href="https://wiki.asterisk.org/wiki/users/editmyemailsettings.action" style="color: #326ca6; text-decoration: none">Manage Notifications</a> </span> </td>
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<td id="email-footer" align="center" style="font-size: 13px; padding: 0 16px 32px 16px; margin: 0"> <small style="font-size: 11px"> This message was sent by <a class="email-footer-link" style="color:#505050;font-size:11px;text-decoration:none;; color: #326ca6; text-decoration: none; color: #505050; font-size: 11px" href="http://www.atlassian.com/software/confluence">Atlassian Confluence</a> 5.1.5, <a class="email-footer-link" style="color:#505050;font-size:11px;text-decoration:none;; color: #326ca6; text-decoration: none; color: #505050; font-size: 11px" href="http://www.atlassian.com/software/confluence/overview/team-collaboration-software?utm_source=email-footer">Team Collaboration Software</a> </small> </td>
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