Use shared memory for moving data over BlobChannel Mojo interface.

blimp/engine/mojo/blob_channel.mojom

Index: blimp/engine/mojo/blob_channel.mojom
diff --git a/blimp/engine/mojo/blob_channel.mojom b/blimp/engine/mojo/blob_channel.mojom
index 0960916fea10655f4e3511b8e2c1684108c08ace..10065ed73538eb4f14aadb584601fb5c2e3f4278 100644
--- a/blimp/engine/mojo/blob_channel.mojom
+++ b/blimp/engine/mojo/blob_channel.mojom
@@ -5,10 +5,10 @@
 module blimp.engine.mojom;
 
 interface BlobChannel {
-  // Stores the blob |id| in the BlobCache.
-  // TODO(kmarshall): Use shared memory for more efficient data transfer, see
-  // crbug.com/614564 .
-  PutBlob(string id, string data);
+  // Stores the blob |id| in the BlobCache, using shared memory to keep
+  // large data payloads out of the Mojo IPC channel.

[Wez, 2016/06/11 00:16:37]

nit: Isn't the use of shared-memory obvious from the function signature?

[Kevin M, 2016/06/16 21:00:38]

It wasn't to maniscalco@, who requested that I add this comment.

[maniscalco, 2016/06/16 21:07:14]

Sorry if I was unclear.  The "why" is the part I'm interested in capturing in
the comment.  Assuming the following is correct, how about something like:

"The Mojo IPC channel is a shared resource and blobs can be quite large.  We
don't want to impact other features/components that use the IPC channel so we
send blobs using shared memory to reduce the message latency experienced by
other components/features."

[Kevin M, 2016/06/21 17:19:46]

How's this?

[maniscalco, 2016/06/21 17:33:15]

That captures it nicely.

+  // The service invokes the no-arg callback after the operation has completed.
+  PutBlob(string id, handle<shared_buffer> data, uint32 size) => ();

[Wez, 2016/06/11 00:16:37]

nit: Do we need |size| to be explicit? Does Mojo not convey the size of the
shared-memory area as part of |data|?

[Kevin M, 2016/06/16 21:00:38]

It doesn't. Handles are just opaque ints, and the memory-mapping functions that
take a handle as a parameter require the size to be provided explicitly. :(

 
   // Requests that the BlobChannel push the blob |id| over the wire.
   // The request will be ignored if the BlobChannel knows that