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From "Andrew Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5950) The DFSClient and DataNode should use shared memory segments to communicate short-circuit information
Date Fri, 28 Feb 2014 21:14:23 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5950?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13916396#comment-13916396

Andrew Wang commented on HDFS-5950:

Nice work, looks like another meaty patch. I didn't do a super-detailed review like last time,
but overall it looks good and I think we should just get this checked in and start hammering
on it.

- Add new config keys to hdfs-default.xml
- Maybe as a follow on, it'd be good to have metrics for # anchorable blocks, # uncache failures
because of anchoring, # of slots and shms, used slots per shm, anything else you can think
of. I think it'll be useful for debugging.

- I like that there's a method named "safetyDance" :)
- Rather than having Hi and Lo longs, if all you want a 128-bit identifier, why not just use
a byte array? It might also be sufficient just to use a single long. If we retry on collisions,
64 bits should be enough.

- Constructor is doing the same precondition check twice

- Is there a potential race in allocSlot, where T1 is loading, T2 is waiting, T1 signals,
T2 wakes up and tries to alloc another one? Since we signal before putting into notFull, this
can happen. Not the end of the world, but maybe sloppy.
- In unregisterClientShmSlot, for the {{stale && empty}} case, would help to drop
a comment about how the shm is unregistered in DfsClientShm#handle, so we can just free when
it empties.
- Can put {{@VisibleForTesting}} on the Visitor interface
- pullSlot isn't very descriptive, maybe allocSlotFromExistingShm?
- In the requestFileDescriptors path, I don't see where {{slot == null}} is handled (when
a DN does not support shm segments). Is there a test for this?

While reviewing, I also wrote some notes suitable for pasting as javadoc if you like:

Manages client short-circuit memory segments on the DataNode.

When a client initiates short-circuit read with a datanode, it requests Each short-circuit
client has a shared memory segment. When a client opens a block for short circuit read, a
new "slot" for the block is allocated in the client's shm segment.

Slots are used to track the state of the block on the both the client and datanode. When a
datanode mlocks a block, the corresponding slots for the blocks are marked as "anchorable".
Anchorable blocks can be safely read via zero-copy read. Doing a ZCR involves increasing the
"anchored" refcount on the slot.

When a DN needs to munlock a block, it needs to first wait for the block to be unanchored
by clients doing ZCR. The DN also marks the block's slots as "unanchorable" to prevent additional
clients from trying to ZCR.

The counterpart fo this class on the client is {@link DfsClientShmManager}.

Flag indicating that the slot is valid, i.e. corresponds to a replica on the DataNode.

This flag is set by the client when it allocates a new Slot. Datanodes never set this flag.

This flag is cleared when the slot is no longer being used, e.g. when the client releases
the corresponding file descriptors or the datanode detects that the client's domain socket
was closed.
Manages short-circuit memory segments for an HDFS client.

Clients are responsible for requesting and releasing shared memory segments used for communicating
with the DataNode. The client will try to allocate new slots in the set of existing segments,
falling back to getting a new segment from the DataNode via {@link DataTransferProtocol#requestShortCircuitFds}.

The counterpart to this class on the DataNode is {@link ShortCircuitRegistry}.

> The DFSClient and DataNode should use shared memory segments to communicate short-circuit
> -----------------------------------------------------------------------------------------------------
>                 Key: HDFS-5950
>                 URL: https://issues.apache.org/jira/browse/HDFS-5950
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: datanode, hdfs-client
>    Affects Versions: 2.4.0
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>         Attachments: HDFS-5950.001.patch, HDFS-5950.003.patch, HDFS-5950.004.patch, HDFS-5950.006.patch,
> The DFSClient and DataNode should use the shared memory segments and unified cache added
in the other HDFS-5182 subtasks to communicate short-circuit information.

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