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authorChristian Schoenebeck <qemu_oss@crudebyte.com>2020-07-29 10:42:28 +0200
committerChristian Schoenebeck <qemu_oss@crudebyte.com>2020-08-12 09:17:32 +0200
commitda9f2eda2551c1cbd98f72730e5b754f2149a85c (patch)
tree48ee855dce65f2d4207708beb6d41252880b7cd3 /hw/9pfs
parentd2c5cf7ca15490b4737a5393e51abf0301b98971 (diff)
9pfs: clarify latency of v9fs_co_run_in_worker()
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> Message-Id: <4d34d332e1aaa8a2cf8dc0b5da4fd7727f2a86e8.1596012787.git.qemu_oss@crudebyte.com> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Diffstat (limited to 'hw/9pfs')
-rw-r--r--hw/9pfs/coth.h12
1 files changed, 11 insertions, 1 deletions
diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h
index fd4a45bc7c..c51289903d 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 { \