Provided by: trafficserver-dev_9.1.3+ds-1_amd64 
NAME
TSContScheduleOnPool - TSContScheduleOnPool API function
SYNOPSIS
#include <ts/ts.h>
TSAction TSContScheduleOnPool(TSCont contp, TSHRTime timeout, TSThreadPool tp)
DESCRIPTION
Mostly the same as TSContSchedule(). Schedules contp on a random thread that belongs to
tp. If thread type of the thread specified by thread affinity is the same as tp, the
contp will be scheduled on the thread specified by thread affinity.
The continuation is scheduled for a particular thread selected from a group of similar
threads, as indicated by tp.
┌────────────────────┬──────────────────────────────────┐
│Pool │ Properties │
├────────────────────┼──────────────────────────────────┤
│TS_THREAD_POOL_NET │ Transaction processing threads. │
│ │ Continuations on these threads │
│ │ must not block. │
├────────────────────┼──────────────────────────────────┤
│TS_THREAD_POOL_TASK │ Background threads. │
│ │ Continuations can perform │
│ │ blocking operations. │
├────────────────────┼──────────────────────────────────┤
│TS_THREAD_POOL_DNS │ DNS request processing. May not │
│ │ exist depending on │
│ │ configuration. Not recommended. │
├────────────────────┼──────────────────────────────────┤
│TS_THREAD_POOL_UDP │ UDP processing. │
└────────────────────┴──────────────────────────────────┘
In practice, any choice except TS_THREAD_POOL_NET or TS_THREAD_POOL_TASK is strongly not
recommended. The TS_THREAD_POOL_NET threads are the same threads on which callback hooks
are called and continuations that use them have the same restrictions. TS_THREAD_POOL_TASK
threads are threads that exist to perform long or blocking actions, although sufficiently
long operation can impact system performance by blocking other continuations on the
threads.
Note that the TSContSchedule() family of API shall only be called from an ATS EThread.
Calling it from raw non-EThreads can result in unpredictable behavior.
EXAMPLE SCENARIOS
Scenario 1 (no thread affinity info, different types of threads)
When thread affinity is not set, a plugin calls the API on thread "A" (which is an
"ET_TASK" type), and wants to schedule on an "ET_NET" type thread provided in "tp", the
system would see there is no thread affinity information stored in "contp."
In this situation, system sees there is no thread affinity information stored in "contp".
It then checks whether the type of thread "A" is the same as provided in "tp", and sees
that "A" is "ET_TASK", but "tp" says "ET_NET". So "contp" gets scheduled on the next
available "ET_NET" thread provided by a round robin list, which we will call thread "B".
Since "contp" doesn't have thread affinity information, thread "B" will be assigned as the
affinity thread for it automatically.
The reason for doing this is most of the time people want to schedule the same things on
the same type of thread, so logically it is better to default the first thread that it is
scheduled on as the affinity thread.
Scenario 2 (no thread affinity info, same types of threads)
Slight variation of scenario 1, instead of scheduling on a "ET_NET" thread, the plugin
wants to schedule on a "ET_TASK" thread (i.e. "tp" contains "ET_TASK" now), all other
conditions stays the same.
This time since the type of the desired thread for scheduling and thread "A" are the same,
the system schedules "contp" on thread "A", and assigns thread "A" as the affinity thread
for "contp".
The reason behind this choice is that we are trying to keep things simple such that lock
contention problems happens less. And for the most part, there is no point of scheduling
the same thing on several different threads of the same type, because there is no
parallelism between them (a thread will have to wait for the previous thread to finish,
either because locking or the nature of the job it's handling is serialized since its on
the same continuation).
Scenario 3 (has thread affinity info, different types of threads)
Slight variation of scenario 1, thread affinity is set for continuation "contp" to thread
"A", all other conditions stays the same.
In this situation, the system sees that the "tp" has "ET_NET", but the type of thread "A"
is "ET_TASK". So even though "contp" has an affinity thread, the system will not use that
information since the type is different, instead it schedules "contp" on the next
available "ET_NET" thread provided by a round robin list, which we will call thread "B".
The difference with scenario 1 is that since thread "A" is set to be the affinity thread
for "contp" already, the system will NOT overwrite that information with thread "B".
Most of the time, a continuation will be scheduled on one type of threads, and rarely gets
scheduled on a different type. But when that happens, we want it to return to the thread
it was previously on, so it won't have any lock contention problems. And that's also why
"thread_affinity" is not a hashmap of thread types and thread pointers.
Scenario 4 (has thread affinity info, same types of threads)
Slight variation of scenario 3, the only difference is "tp" now says "ET_TASK".
This is the easiest scenario since the type of thread "A" and "tp" are the same, so the
system schedules "contp" on thread "A". And, as discussed, there is really no reason why
one may want to schedule the same continuation on two different threads of the same type.
NOTE:
In scenario 3 & 4, it doesn't matter which thread the plugin is calling the API from.
SEE ALSO
TSContSchedule TSContScheduleEvery TSContScheduleOnThread
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