From edcfea9753bd31d2cf2ebec6fe5f46ad763f7cc3 Mon Sep 17 00:00:00 2001 From: Ned Bass Date: Fri, 7 Jun 2013 15:22:34 -0700 Subject: [PATCH] LU-3443 mdc: document mdc_rpc_lock As this lock can be a bottleneck, clarifying why it is needed may be helpful to those working on client performance. Signed-off-by: Ned Bass Change-Id: Ib617a99a51b65b1141e52a6f31d4431828203650 Reviewed-on: http://review.whamcloud.com/6593 Tested-by: Hudson Reviewed-by: Andreas Dilger Reviewed-by: Keith Mannthey Tested-by: Maloo --- lustre/include/lustre_mdc.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/lustre/include/lustre_mdc.h b/lustre/include/lustre_mdc.h index 6cb5f01..bc04678 100644 --- a/lustre/include/lustre_mdc.h +++ b/lustre/include/lustre_mdc.h @@ -69,9 +69,27 @@ struct obd_export; struct ptlrpc_request; struct obd_device; +/** + * Serializes in-flight MDT-modifying RPC requests to preserve idempotency. + * + * This mutex is used to implement execute-once semantics on the MDT. + * The MDT stores the last transaction ID and result for every client in + * its last_rcvd file. If the client doesn't get a reply, it can safely + * resend the request and the MDT will reconstruct the reply being aware + * that the request has already been executed. Without this lock, + * execution status of concurrent in-flight requests would be + * overwritten. + * + * This design limits the extent to which we can keep a full pipeline of + * in-flight requests from a single client. This limitation could be + * overcome by allowing multiple slots per client in the last_rcvd file. + */ struct mdc_rpc_lock { + /** Lock protecting in-flight RPC concurrency. */ struct mutex rpcl_mutex; + /** Intent associated with currently executing request. */ struct lookup_intent *rpcl_it; + /** Used for MDS/RPC load testing purposes. */ int rpcl_fakes; }; -- 1.8.3.1