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| author | Christian Schoenebeck <qemu_oss@crudebyte.com> | 2020-07-29 10:42:28 +0200 |
|---|---|---|
| committer | Christian Schoenebeck <qemu_oss@crudebyte.com> | 2020-08-12 09:17:32 +0200 |
| commit | da9f2eda2551c1cbd98f72730e5b754f2149a85c (patch) | |
| tree | 48ee855dce65f2d4207708beb6d41252880b7cd3 /hw/9pfs | |
| parent | d2c5cf7ca15490b4737a5393e51abf0301b98971 (diff) | |
| download | focaccia-qemu-da9f2eda2551c1cbd98f72730e5b754f2149a85c.tar.gz focaccia-qemu-da9f2eda2551c1cbd98f72730e5b754f2149a85c.zip | |
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.h | 12 |
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 { \ |