Optimize read/take path with SHM when waitset is not used for the data notification #1173

sumanth-nirmal · 2022-03-02T17:44:44Z

This MR changes the way the iceoryx listener is attached to the reader to avoid additional thread context to update the data from the iox_queue to the reader history cache when publish and subscribe happens locally without the waitset.

With the changes, the data updates happen differently as in the following scenarios

Scenario-1
(SHM enabled)
A setup where data is published, and data is read/taken without a waitset or any data available notification mechanisms, then the iox_listener is not attached to the reader, which means the data from iox queue is transferred to the reader history cache in the same context (with out any additional thread context).

Scenario-2
(SHM enabled)
A setup where data is published and data is read/taken by waiting on a waitset with any data availability status mask or
Read condition, then the iox_listener is attached to this reader, which means the data transfer from iox queue to the reader history cache happens in an additional background thread.

This needs to be merged after #1175

MatthiasKillat

@sumanth-nirmal Looks ok in general, but there are minor changes required. The status handling in the waitset is unclear to me.

src/core/ddsc/src/dds_read.c

MatthiasKillat · 2022-03-03T12:10:55Z

src/core/ddsc/src/dds_read.c

+    enum iox_ChunkReceiveResult take_result = iox_sub_take_chunk(rd->m_iox_sub, (const void **const)&chunk);
+    shm_unlock_iox_sub(rd->m_iox_sub);
+
+    if (ChunkReceiveResult_SUCCESS != take_result)


Here we may need to distinguish between various different errors like

enum iox_ChunkReceiveResult { ChunkReceiveResult_TOO_MANY_CHUNKS_HELD_IN_PARALLEL, ChunkReceiveResult_NO_CHUNK_AVAILABLE, ChunkReceiveResult_UNDEFINED_ERROR, ChunkReceiveResult_SUCCESS, };

to e.g. warn a user that we hold to many chunks in parallel (meaning the consumer working with the chunks is too slow and we hence may lose chunks).

I can do this in an upcoming PR which establishes compatibility with iceoryx 2.0 (cf. #1174)

I have abstracted the code from shm_monitor to a separate API for this functionality, so the error handling should be good.

MatthiasKillat · 2022-03-03T12:19:24Z

src/core/ddsc/src/dds_read.c

+    // Get writer or proxy writer
+    struct entity_common *e = entidx_lookup_guid_untyped(gv->entity_index, &ice_hdr->guid);
+    if (e == NULL || (e->kind != EK_PROXY_WRITER && e->kind != EK_WRITER)) {
+      // Ignore that doesn't match a known writer or proxy writer


missing part in comment: that what does not match? Comment is unclear to me. I think we check whether we know the writer/sender of the data.

src/core/ddsc/src/dds__reader.h

MatthiasKillat · 2022-03-03T12:31:22Z

src/core/ddsc/src/dds_reader.c

@@ -800,6 +794,13 @@ uint32_t dds_reader_lock_samples (dds_entity_t reader)
  uint32_t n;
  if (dds_reader_lock (reader, &rd) != DDS_RETCODE_OK)
    return 0;
+#ifdef DDS_HAS_SHM


Not sure whether this is the right place (logically) to pull the samples from iceoryx to the rhc here.

This is purely by name of the function, which indicates that somehow the rhc is locked but what else happens? Maybe it is more like an update of the rhc, but such an update must happen protected by a lock (i.e. this is the right place but the name of the function is off).

This is the entry point for the C++ API, it first calls this function to check the number of samples in the reader history cache, then updates the buffers accordingly, and then calls the actual read/take_cdr API. So in the context of C++ API I guess this should be correct.

WRT C API, I have added this in read_take_impl functions, which I need to check with @eboasson if it makes sense

MatthiasKillat · 2022-03-03T12:53:50Z

src/core/ddsc/src/shm_monitor.c

+            return DDS_RETCODE_OUT_OF_RESOURCES;
+        }
+
+        // TODO(Sumanth), do we even use this at all?


I think the iox listener does sufficient bookkeeping of this on its own, so unless we want a lower maximum of attachments than the listener allows or want to query the number of attachments we will not need it.

I will remove this.

MatthiasKillat · 2022-03-03T12:56:07Z

src/core/ddsc/src/shm_monitor.c

+        // TODO(Sumanth), do we even use this at all?
+        ++monitor->m_number_of_attached_readers;
+        reader->m_iox_sub_stor.monitor = &reader->m_entity.m_domain->m_shm_monitor;
+        reader->m_iox_sub_stor.parent_reader = reader;


Race: must be set before attaching, since as soon as it is attached the callback may try to access t.

This means if attaching fails it could be reset, but technically this could also be set once on init of the reader after the iceoryx subscriber is created as it does not change.

I am setting now this in the init

MatthiasKillat · 2022-03-03T12:57:01Z

src/core/ddsc/src/shm_monitor.c

    }
-    ++monitor->m_number_of_attached_readers;
-
+    ddsrt_mutex_unlock(&monitor->m_lock);


I think this is not needed (see locking above).

MatthiasKillat · 2022-03-03T12:57:58Z

src/core/ddsc/src/shm_monitor.c

+        // are we really tracking the number of attached readers?
+        --monitor->m_number_of_attached_readers;
+        reader->m_iox_sub_stor.monitor = NULL;
+        reader->m_iox_sub_stor.parent_reader = NULL;


Not needed on every attach/detach cyclone, setting it once to non-null is enough.

MatthiasKillat · 2022-03-03T12:58:54Z

src/core/ddsc/src/shm_monitor.c

+        iox_listener_detach_subscriber_event(monitor->m_listener, reader->m_iox_sub, SubscriberEvent_DATA_RECEIVED);
+        // are we really tracking the number of attached readers?
+        --monitor->m_number_of_attached_readers;
+        reader->m_iox_sub_stor.monitor = NULL;


It is probably better to unset this (i.e. set to NULL) after detaching, but I think it is technically not required.
Previously it was done in a way so that there should be no races in the callback the listener executes. I think after the proposed changes this should not cause races as well.

MatthiasKillat · 2022-03-11T11:01:22Z

@sumanth-nirmal Had a brief look again, will review today once #1175 is merged.

MatthiasKillat · 2022-03-11T16:54:50Z

src/core/ddsc/src/dds_read.c

+
+  while (true)
+  {
+    shm_lock_iox_sub(rd->m_iox_sub);


It may also worth looking into whether it pays of to lock for the whole while loop (longer critical section but less lock/unlock).

MatthiasKillat · 2022-03-11T17:04:56Z

src/core/ddsc/src/dds_waitset.c

+        // we fail if we cannot attach to the listener (as we would get no data)
+        iox_sub_deinit(rd->m_iox_sub);
+        rd->m_iox_sub = NULL;
+        ret = DDS_RETCODE_ERROR;


Ok, we may now fail if the iceoryx listener fails to attach (due to resource limits). But previously we would certainly have failed at reader creation (and since not every reader will use a waitset we will now not necessarily fail).

MatthiasKillat · 2022-03-11T17:06:08Z

src/core/ddsc/src/dds_waitset.c

+        struct dds_reader * rd = (dds_reader *) e->m_parent;
+        if (rd->m_iox_sub != NULL) {
+          // if there are no valid statuses
+          if (!dds_entity_supports_validate_status(e)) {


I see and it sounds reasonable. But I do not know whether there is more to it.

MatthiasKillat · 2022-03-11T17:10:02Z

src/core/ddsc/src/shm_monitor.c

+    // if the reader is attached
+    if (reader->m_iox_sub_context.monitor != NULL && reader->m_iox_sub_context.parent_reader != NULL) {
+        iox_listener_detach_subscriber_event(monitor->m_listener, reader->m_iox_sub, SubscriberEvent_DATA_RECEIVED);
+        reader->m_iox_sub_context.monitor = NULL;


OK.
There should be no race in the callback due to setting these NULL, as it should run to completion before detaching. This also means a blocking callback will block detach, but this was also the case before (and the callback is not blocking).

MatthiasKillat · 2022-03-11T17:13:39Z

src/core/ddsc/src/shm_monitor.c

-    if(storage->monitor->m_state == SHM_MONITOR_RUNNING && storage->call) {
-        storage->call(storage->arg);
-    }
+  dds_transfer_samples_from_iox_to_rhc(rd);


I think we can now call this directly in shm_subscriber_callback, i.e. there is no need for receive_data_wakeup_handler anymore.

MatthiasKillat · 2022-03-11T17:14:19Z

src/core/ddsc/src/shm_monitor.c

-        receive_data_wakeup_handler(storage->parent_reader);
+    iox_sub_context_t *context = (iox_sub_context_t*) context_data;
+    if((context->monitor) && (context->monitor->m_state == SHM_MONITOR_RUNNING)) {
+        receive_data_wakeup_handler(context->parent_reader);


We can call dds_transfer_samples_from_iox_to_rhc(context->parent_reader) here.

MatthiasKillat · 2022-03-11T17:23:56Z

Pulling the data from iceoryx in a read is beneficial to avoid context switches (icoeryx listener thread). As long as the buffer of iceoryx is at least as large as the reader history cache, there is no data loss that we would not experience otherwise.

If the user is actively waiting for data the listener works as before. I wonder if this could be optimized for the DDS waitset to also only pull data in the read call after being woken up by the waitset? Even for a listener with background thread this would work, as long as it calls the regular read (which pulls data from iceoryx as needed).

I was also thinking of whether we could have some kind of hybrid approach:

attach a listener, observe data events but more specific ones like: there are N samples in the iceoryx buffer (configurable, say N is half the cache size).
only wake up if we have N samples an transfer them.
transfer in any read as well, regardless of number of samples in iceoryx buffer

Unfortunately iceoryx lacks the trigger event for this (waking up on any sample and checking the number defeats the purpose of having less wake-ups). Therefore it is not possible but we may change iceoryx for this in the future.

- Make them thread safe - The attach API will attach only if the reader is not already attached - The attach API also updates the iox_sub storage with monitor and the parent reader - The detach API will only attempt to detach if there is a monitor attched Signed-off-by: Sumanth Nirmal <[email protected]>

…ory cache from shm_monitor.c to dds_read.c Signed-off-by: Sumanth Nirmal <[email protected]>

…scriber queue to the reader cache directly in the same thread context, if the monitor is not attached to this reader Signed-off-by: Sumanth Nirmal <[email protected]>

The reader will be attached to the monitor when the reader has read conditions or status conditions or listener with data availability status mask Signed-off-by: Sumanth Nirmal <[email protected]>

Signed-off-by: Sumanth Nirmal <[email protected]>

read/take_impl APIs when SHM is supported and iox_listener is not attached to a specific reader Signed-off-by: Sumanth Nirmal <[email protected]>

Signed-off-by: Sumanth Nirmal <[email protected]>

MatthiasKillat reviewed Mar 3, 2022

View reviewed changes

sumanth-nirmal force-pushed the decouple-listener-thread-for-take-optimization branch 7 times, most recently from 945ab91 to 05eae64 Compare March 8, 2022 02:38

sumanth-nirmal marked this pull request as ready for review March 8, 2022 07:32

sumanth-nirmal force-pushed the decouple-listener-thread-for-take-optimization branch 2 times, most recently from 7249200 to fa19754 Compare March 11, 2022 07:53

MatthiasKillat reviewed Mar 11, 2022

View reviewed changes

sumanth-nirmal force-pushed the decouple-listener-thread-for-take-optimization branch 3 times, most recently from 47547b3 to c570ab3 Compare March 17, 2022 22:00

sumanth-nirmal added 10 commits March 24, 2022 06:14

Abstract the method to transfer samples from iox queue to reader hist…

45d96da

…ory cache from shm_monitor.c to dds_read.c Signed-off-by: Sumanth Nirmal <[email protected]>

Update dds_reader_lock_samples to update the samples from the iox sub…

7fa881d

…scriber queue to the reader cache directly in the same thread context, if the monitor is not attached to this reader Signed-off-by: Sumanth Nirmal <[email protected]>

Remove the logic of attaching reader to shm monitor for every reader

1637298

The reader will be attached to the monitor when the reader has read conditions or status conditions or listener with data availability status mask Signed-off-by: Sumanth Nirmal <[email protected]>

Update waitset attach method to attach the reader to the shm monitor

9d79e03

Signed-off-by: Sumanth Nirmal <[email protected]>

Update waitset detach method to detach the reader from the shm monitor

6bad2ce

Signed-off-by: Sumanth Nirmal <[email protected]>

Remove the logic of tracking attached iox listeners

bbaab3b

Signed-off-by: Sumanth Nirmal <[email protected]>

Remove the unused mutex in shm_monitor

6739df2

Signed-off-by: Sumanth Nirmal <[email protected]>

Update samples from iox_queue to reader history cache in the

23429fc

read/take_impl APIs when SHM is supported and iox_listener is not attached to a specific reader Signed-off-by: Sumanth Nirmal <[email protected]>

Resolve merge conflicts

bbbb793

Signed-off-by: Sumanth Nirmal <[email protected]>

sumanth-nirmal force-pushed the decouple-listener-thread-for-take-optimization branch from c570ab3 to bbbb793 Compare March 24, 2022 13:14

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Optimize read/take path with SHM when waitset is not used for the data notification #1173

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Optimize read/take path with SHM when waitset is not used for the data notification #1173

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