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https://issues.apache.org/jira/browse/TS-4316?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15475544#comment-15475544
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Masakazu Kitajo commented on TS-4316:
-------------------------------------
{quote}
The question is not whether this occurs, but how often. Is this rare or common?
{quote}
I don't know how often it occurs and how much it affects performance. I just
realized this can be occurred, and I thought it has negative effect.
{quote}
If the slot index accelerators where changed to be 8 bits, then only headers
stored in slots past 253 would need to be searched, which should be large
enough for quite some time. This would add only 16 bytes per alternate.
{quote}
I agree. The percentage of unlucky scenario would be much decreased.
2 / 16 = 0.125
2 / 256 = 0.0078125
> 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: sometime
>
>
> 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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