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https://issues.apache.org/jira/browse/TS-4316?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15469832#comment-15469832
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Masakazu Kitajo commented on TS-4316:
-------------------------------------
I agree that changing {{MIME_FIELD_SLOTNUM_UNKNOWN}} to 16 would need quite a
big changes. Also we might not be able to use 15 as well.
However, the problem is that slot 14 and 15 (the actual places, not the
numbers) are used to store fields when we attach fields. But we cannot access
there to get the fields because 14 and 15 have special meanings. So, if 14 and
15 are invalid as normal slot numbers, I think we shouldn't store any fields to
the slots and we should find the next really available slot.
> Slot accelerator doesn't effect under certain condition
> -------------------------------------------------------
>
> Key: TS-4316
> URL: https://issues.apache.org/jira/browse/TS-4316
> Project: Traffic Server
> Issue Type: Bug
> Components: MIME
> Reporter: Masakazu Kitajo
> Assignee: Alan M. Carroll
> Fix For: 7.0.0
>
>
> Slot accelerators seem to not effect if the stored slot number is 14 or 15.
> This is because {{mime_hdr_set_accelerators_and_presence_bits()}} regards
> slot number 14+ as "unknown" and stores MIME_FIELD_SLOTNUM_UNKNOWN(14) to an
> accelerator, and it causes slower search. Although there is no critical
> effect, it would decreases performance slightly.
> The numbers, 14 and 15, comes from constants below in MIME.h:
> {code}
> #define MIME_FIELD_SLOTNUM_BITS 4
> #define MIME_FIELD_SLOTNUM_MASK ((1 << MIME_FIELD_SLOTNUM_BITS) - 1)
> #define MIME_FIELD_SLOTNUM_MAX (MIME_FIELD_SLOTNUM_MASK - 1)
> #define MIME_FIELD_SLOTNUM_UNKNOWN MIME_FIELD_SLOTNUM_MAX
> {code}
> MIME_FIELD_SLOTNUM_MASK is 15.
> MIME_FIELD_SLOTNUM_MAX is 14.
> MIME_FIELD_SLOTNUM_UNKNOWN is 14 too.
> Though it still needs more careful confirmation, it seems we can simply
> change {{MIME_FIELD_SLOTNUM_UNKNOWN}} to the value of
> {{MIME_FIELD_SLOTNUM_MASK}} (15). But we still cannot use 15.
> To use 15 as a valid slot number in slot accelerators, we need to change
> {{MIME_FIELD_SLOTNUM_UNKNOWN}} to 16 or -1, however it would need to change
> some codes also (not only the number).
> This bug has been found while I was tackling TS-4313.
> https://github.com/apache/trafficserver/pull/542#discussion-diff-58226891
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