When a single thread is reading from a libaio io_context_t object
in a non-blocking polling manner (that is, with the minimum number
of events to return being 0), then it is possible to safely read
events directly from user-space, taking advantage of the fact that
the io_context_t object is a pointer to memory with a certain layout.
This patch adds an option, userspace_libaio_reap, which allows
reading events in this manner when the libaio engine is used.
You can observe its effect by setting iodepth_batch_complete=0
and seeing the change in distribution of system/user time based on
whether this new flag is set. If userspace_libaio_reap=1, then
busy polling takes place in userspace, and there is a larger amount of
usr CPU. If userspace_libaio_reap=0 (the default), then there is a
larger amount of sys CPU from the polling in the kernel.
Polling from a queue in this manner is several times faster. In my
testing, it took less than an eighth as much time to execute a
polling operation in user-space than with the io_getevents syscall.
---
HOWTO | 7 +++++++
engines/libaio.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++++-
fio.h | 2 ++
options.c | 9 +++++++++
4 files changed, 69 insertions(+), 1 deletions(-)
---
Version 2 updates the HOWTO file and uses the necessary read barrier.
I am not sure if this needs a flush_dcache_page call in the kernel
for correctness. A future version should consider cleaning up the
option interface and consider moving the reaping code to libaio.
diff --git a/HOWTO b/HOWTO
index a1b2e8c..ad4e454 100644
--- a/HOWTO
+++ b/HOWTO
@@ -1187,6 +1187,13 @@ uid=int Instead of running as the invoking user, set the user ID to
gid=int Set group ID, see uid.
+userspace_libaio_reap=bool Normally, with the libaio engine in use, fio
+ will use the io_getevents system call to reap newly returned
+ events. With this flag turned on, the AIO ring will be read
+ directly from user-space to reap events. The reaping mode is
+ only enabled when polling for a minimum of 0 events (eg when
+ iodepth_batch_complete=0).
+
6.0 Interpreting the output
---------------------------
diff --git a/engines/libaio.c b/engines/libaio.c
index c837ab6..ea05c63 100644
--- a/engines/libaio.c
+++ b/engines/libaio.c
@@ -58,6 +58,47 @@ static struct io_u *fio_libaio_event(struct thread_data *td, int event)
return io_u;
}
+struct aio_ring {
+ unsigned id; /** kernel internal index number */
+ unsigned nr; /** number of io_events */
+ unsigned head;
+ unsigned tail;
+
+ unsigned magic;
+ unsigned compat_features;
+ unsigned incompat_features;
+ unsigned header_length; /** size of aio_ring */
+
+ struct io_event events[0];
+};
+
+#define AIO_RING_MAGIC 0xa10a10a1
+
+static int user_io_getevents(io_context_t aio_ctx, unsigned int max,
+ struct io_event *events)
+{
+ long i = 0;
+ unsigned head;
+ struct aio_ring *ring = (struct aio_ring*)aio_ctx;
+
+ while (i < max) {
+ head = ring->head;
+
+ if (head == ring->tail) {
+ /* There are no more completions */
+ break;
+ } else {
+ /* There is another completion to reap */
+ events[i] = ring->events[head];
+ read_barrier();
+ ring->head = (head + 1) % ring->nr;
+ i++;
+ }
+ }
+
+ return i;
+}
+
static int fio_libaio_getevents(struct thread_data *td, unsigned int min,
unsigned int max, struct timespec *t)
{
@@ -66,7 +107,16 @@ static int fio_libaio_getevents(struct thread_data *td, unsigned int min,
int r, events = 0;
do {
- r = io_getevents(ld->aio_ctx, actual_min, max, ld->aio_events + events, t);
+ if (td->o.userspace_libaio_reap == 1
+ && actual_min == 0
+ && ((struct aio_ring *)(ld->aio_ctx))->magic
+ == AIO_RING_MAGIC) {
+ r = user_io_getevents(ld->aio_ctx, max,
+ ld->aio_events + events);
+ } else {
+ r = io_getevents(ld->aio_ctx, actual_min,
+ max, ld->aio_events + events, t);
+ }
if (r >= 0)
events += r;
else if (r == -EAGAIN)
diff --git a/fio.h b/fio.h
index 9d2a61c..0c86f28 100644
--- a/fio.h
+++ b/fio.h
@@ -413,6 +413,8 @@ struct thread_options {
unsigned int gid;
unsigned int sync_file_range;
+
+ unsigned int userspace_libaio_reap;
};
#define FIO_VERROR_SIZE 128
diff --git a/options.c b/options.c
index 6a87e98..6f7c41e 100644
--- a/options.c
+++ b/options.c
@@ -2069,6 +2069,15 @@ static struct fio_option options[FIO_MAX_OPTS] = {
.off1 = td_var_offset(gid),
.help = "Run job with this group ID",
},
+#ifdef FIO_HAVE_LIBAIO
+ {
+ .name = "userspace_libaio_reap",
+ .type = FIO_OPT_BOOL,
+ .off1 = td_var_offset(userspace_libaio_reap),
+ .help = "When using the libaio engine with iodepth_batch_complete=0, enable userspace reaping",
+ .def = "0",
+ },
+#endif
{
.name = NULL,
},
--
1.7.3.1
--
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