Improve accuracy of rate= option

I noticed that the rate= option is not terribly precise in some cases.
 It gets worse the higher the selected rate.  For example:
$ fio -name=load -size=100g -ioengine=null -runtime=10 -rate=30m
One would expect that to read 300MB (307200KB) at close to 30MB/s
(30720KB/s).  However it writes 315024KB at 31499KB/s.  Further
experimentation shows that even higher rates can show bigger
discrepancies.  At the extreme end...
$ fio -name=load -size=100g -ioengine=null -runtime=10 -rate=500m
One would expect this to write 5000MB at a rate of 500MB/s
(512000KB/s).  However it writes close to double that (9536.8MB) at a
rate of over 953MB/s.  At a rate of 1GB/s and higher, the rate
limiting is effectively ignored.

This patch improves the accuracy of the rate= option across the whole
range of rates, at the cost of being very slightly more
computationally expensive.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fio.h b/fio.h
index be684ca2544e4c9c5fce8e1655dd2d8a3739535f..a5405e375da7486cbf8beb687c19ba313130ea30 100644 (file)
--- a/fio.h
+++ b/fio.h
@@ -357,7 +357,7 @@ struct thread_data {
        /*
         * Rate state
         */
-       unsigned long rate_nsec_cycle[2];
+       unsigned long long rate_bps[2];
        long rate_pending_usleep[2];
        unsigned long rate_bytes[2];
        unsigned long rate_blocks[2];

diff --git a/init.c b/init.c
index 9fafadf5aa1b2fb2bfceb282464fb643f12b19b4..01e4371814b471684117faaaec3e5cb6fea5f707 100644 (file)
--- a/init.c
+++ b/init.c
@@ -327,21 +327,19 @@ static void put_job(struct thread_data *td)
 static int __setup_rate(struct thread_data *td, enum fio_ddir ddir)
 {
        unsigned int bs = td->o.min_bs[ddir];
-       unsigned long long bytes_per_sec;
 
        assert(ddir_rw(ddir));
 
        if (td->o.rate[ddir])
-               bytes_per_sec = td->o.rate[ddir];
+               td->rate_bps[ddir] = td->o.rate[ddir];
        else
-               bytes_per_sec = td->o.rate_iops[ddir] * bs;
+               td->rate_bps[ddir] = td->o.rate_iops[ddir] * bs;
 
-       if (!bytes_per_sec) {
+       if (!td->rate_bps[ddir]) {
                log_err("rate lower than supported\n");
                return -1;
        }
 
-       td->rate_nsec_cycle[ddir] = 1000000000ULL / bytes_per_sec;
        td->rate_pending_usleep[ddir] = 0;
        return 0;
 }

diff --git a/io_u.c b/io_u.c
index fc3ee498e2b497d9d410970c1427bc06006e97d2..0ff66f9dc4036402e0030c06f1aa17cebdc7c110 100644 (file)
--- a/io_u.c
+++ b/io_u.c
@@ -1296,6 +1296,16 @@ static void account_io_completion(struct thread_data *td, struct io_u *io_u,
        add_iops_sample(td, idx, &icd->time);
 }
 
+static long long usec_for_io(struct thread_data *td, enum fio_ddir ddir)
+{
+       unsigned long long secs, remainder, bps, bytes;
+       bytes = td->this_io_bytes[ddir];
+       bps = td->rate_bps[ddir];
+       secs = bytes / bps;
+       remainder = bytes % bps;
+       return remainder * 1000000 / bps + secs * 1000000;
+}
+
 static void io_completed(struct thread_data *td, struct io_u *io_u,
                         struct io_completion_data *icd)
 {
@@ -1354,14 +1364,12 @@ static void io_completed(struct thread_data *td, struct io_u *io_u,
 
                        if (__should_check_rate(td, idx)) {
                                td->rate_pending_usleep[idx] =
-                                       ((td->this_io_bytes[idx] *
-                                         td->rate_nsec_cycle[idx]) / 1000 -
+                                       (usec_for_io(td, idx) -
                                         utime_since_now(&td->start));
                        }
-                       if (__should_check_rate(td, idx ^ 1))
+                       if (__should_check_rate(td, odx))
                                td->rate_pending_usleep[odx] =
-                                       ((td->this_io_bytes[odx] *
-                                         td->rate_nsec_cycle[odx]) / 1000 -
+                                       (usec_for_io(td, odx) -
                                         utime_since_now(&td->start));
                }