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From: Christian Schoenebeck <qemu_oss@crudebyte.com>
To: qemu-devel@nongnu.org
Cc: Greg Kurz <groug@kaod.org>
Subject: [PATCH v5 6/6] 9pfs: clarify latency of v9fs_co_run_in_worker()
Date: Thu, 26 Mar 2020 22:25:32 +0100
Message-ID: <7c4e49a783c7b2f77aa81794892cf08b4feba914.1585258105.git.qemu_oss@crudebyte.com> (raw)
In-Reply-To: <cover.1585258105.git.qemu_oss@crudebyte.com>

As we just fixed a severe performance issue with Treaddir request
handling, clarify this overall issue as a comment on
v9fs_co_run_in_worker() with the intention to hopefully prevent
such performance mistakes in future (and fixing other yet
outstanding ones).

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
---
 hw/9pfs/coth.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h
index a6851822d5..8b6f76840a 100644
--- a/hw/9pfs/coth.h
+++ b/hw/9pfs/coth.h
@@ -19,7 +19,7 @@
 #include "qemu/coroutine.h"
 #include "9p.h"
 
-/*
+/**
  * we want to use bottom half because we want to make sure the below
  * sequence of events.
  *
@@ -28,6 +28,16 @@
  *   3. Enter the coroutine in the worker thread.
  * we cannot swap step 1 and 2, because that would imply worker thread
  * can enter coroutine while step1 is still running
+ *
+ * @b PERFORMANCE @b CONSIDERATIONS: As a rule of thumb, keep in mind
+ * that hopping between threads adds @b latency! So when handling a
+ * 9pfs request, avoid calling v9fs_co_run_in_worker() too often, because
+ * this might otherwise sum up to a significant, huge overall latency for
+ * providing the response for just a single request. For that reason it
+ * is highly recommended to fetch all data from fs driver with a single
+ * fs driver request on a background I/O thread (bottom half) in one rush
+ * first and then eventually assembling the final response from that data
+ * on main I/O thread (top half).
  */
 #define v9fs_co_run_in_worker(code_block)                               \
     do {                                                                \
-- 
2.20.1



      parent reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-26 21:28 [PATCH v5 0/6] 9pfs: readdir optimization Christian Schoenebeck
2020-03-26 21:24 ` [PATCH v5 1/6] tests/virtio-9p: added split readdir tests Christian Schoenebeck
2020-03-26 21:24 ` [PATCH v5 2/6] 9pfs readdir: rename max_count -> maxsize Christian Schoenebeck
2020-03-26 21:24 ` [PATCH v5 3/6] 9pfs: make v9fs_readdir_response_size() public Christian Schoenebeck
2020-03-26 21:25 ` [PATCH v5 4/6] 9pfs: add new function v9fs_co_readdir_many() Christian Schoenebeck
2020-03-26 21:25 ` [PATCH v5 5/6] 9pfs: T_readdir latency optimization Christian Schoenebeck
2020-03-26 21:25 ` Christian Schoenebeck [this message]

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