[jira] [Updated] (TS-3768) Implement a tiered back-off in read-while-writer retries

["Bryan Call (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/TS-3768?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Bryan Call updated TS-3768:
---------------------------
    Summary: Implement a tiered back-off in read-while-writer retries  (was: Implement a tiered back-off in read-while-writer retries.)

> Implement a tiered back-off in read-while-writer retries
> --------------------------------------------------------
>
>                 Key: TS-3768
>                 URL: https://issues.apache.org/jira/browse/TS-3768
>             Project: Traffic Server
>          Issue Type: Bug
>          Components: Cache
>            Reporter: Sudheer Vinukonda
>             Fix For: sometime
>
>
> There's some code in the current cache layer that attempts to do a tiered back off in reattempting read-while-writer (either for acquiring writer lock or waiting for the first fragment download etc), but, that code is buggy (see below).
> https://github.com/apache/trafficserver/blob/master/iocore/cache/P_CacheInternal.h#L100
> {code}
> #define VC_SCHED_WRITER_RETRY()                                   \
>   do {                                                            \
>     ink_assert(!trigger);                                         \
>     writer_lock_retry++;                                          \
>     ink_hrtime _t = WRITER_RETRY_DELAY;                           \
>     if (writer_lock_retry > 2)                                    \
>       _t = WRITER_RETRY_DELAY * 2;                                \
>     else if (writer_lock_retry > 5)                               \
>       _t = WRITER_RETRY_DELAY * 10;                               \
>     else if (writer_lock_retry > 10)                              \
>       _t = WRITER_RETRY_DELAY * 100;                              \
>     trigger = mutex->thread_holding->schedule_in_local(this, _t); \
>     return EVENT_CONT;                                            \
>   } while (0)
> {code}
> Presently, the retry happens as follows: 
> {{WRITER_RETRY_DELAY, WRITER_RETRY_DELAY, 2*WRITER_RETRY_DELAY, 2*WRITER_RETRY_DELAY, 2*WRITER_RETRY_DELAY etc}}
> I'm removing the buggy code with TS-3767 to avoid confusion, but, opening this jira per [~zwoop]'s suggestion to implement a proper tiered back off in the future.



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