Misc throttle fixes

The level of the burst bucket is stored in bkt.burst_level, not bkt.burst_lenght.

Signed-off-by Alberto Garcia <[email protected]>

---
include/qemu/throttle.h | 2 +-

1 file changed, 1 insertion(+), 1 deletion(-)

include/qemu/throttle.h +1 -1

index d056008c18..66a8ac10a4 100644
@@ -63,7 +63,7 @@ typedef enum {
 * - The bkt.avg rate does not apply until the bucket is full,
 *   allowing the user to do bursts until then. The I/O limit during
 *   bursts is bkt.max. To enforce this limit we keep an additional
- *   bucket in bkt.burst_length that leaks at a rate of bkt.max units
+ *   bucket in bkt.burst_level that leaks at a rate of bkt.max units
 *   per second.
 *
 * - Because of all of the above, the user can perform I/O at a

Signature · 1 line

2.11.0

Quoted 2 lines · Alberto Garcia

The level of the burst bucket is stored in bkt.burst_level, not bkt.burst_lenght.

s/lenght/length, otherwise:

Reviewed-by Manos Pitsidianakis <[email protected]>

The throttling code can change internally the value of bkt->max if it hasn't been set by the user. The problem with this is that if we want to retrieve the original value we have to undo this change first. This is ugly and unnecessary: this patch removes the throttle_fix_bucket() and throttle_unfix_bucket() functions completely and moves the logic to throttle_compute_wait().

Signed-off-by Alberto Garcia <[email protected]>

---
util/throttle.c | 62 +++++++++++++++++++++------------------------------------

1 file changed, 23 insertions(+), 39 deletions(-)

util/throttle.c +2 -0

index 9a6bda813c..1f29cf9918 100644
@@ -95,23 +95,36 @@ static int64_t throttle_do_compute_wait(double limit, double extra)
 int64_t throttle_compute_wait(LeakyBucket *bkt)
 {
    double extra; /* the number of extra units blocking the io */
+    double bucket_size;   /* I/O before throttling to bkt->avg */
+    double burst_bucket_size; /* Before throttling to bkt->max */
 
    if (!bkt->avg) {
        return 0;

util/throttle.c +3 -32

        if (extra > 0) {
            return throttle_do_compute_wait(bkt->max, extra);
        }

@@ -358,31 +371,6 @@ bool throttle_is_valid(ThrottleConfig *cfg, Error **errp)
    return true;
 }
 
-/* fix bucket parameters */
-static void throttle_fix_bucket(LeakyBucket *bkt)
-{
-    double min;
-
-    /* zero bucket level */
-    bkt->level = bkt->burst_level = 0;
-
-    /* If bkt->max is 0 we still want to allow short bursts of I/O
-     * from the guest, otherwise every other request will be throttled
-     * and performance will suffer considerably. */
-    min = bkt->avg / 10;
-    if (bkt->avg && !bkt->max) {
-        bkt->max = min;
-    }
-}
-
-/* undo internal bucket parameter changes (see throttle_fix_bucket()) */
-static void throttle_unfix_bucket(LeakyBucket *bkt)
-{
-    if (bkt->max < bkt->avg) {
-        bkt->max = 0;
-    }
-}
-
 /* Used to configure the throttle
 *
 * @ts: the throttle state we are working on

@@ -397,8 +385,10 @@ void throttle_config(ThrottleState *ts,
 
    ts->cfg = *cfg;
 
+    /* Zero bucket level */
    for (i = 0; i < BUCKETS_COUNT; i++) {
-        throttle_fix_bucket(&ts->cfg.buckets[i]);
+        ts->cfg.buckets[i].level = 0;
+        ts->cfg.buckets[i].burst_level = 0;
    }
 
    ts->previous_leak = qemu_clock_get_ns(clock_type);

@@ -411,13 +401,7 @@ void throttle_config(ThrottleState *ts,
 */
 void throttle_get_config(ThrottleState *ts, ThrottleConfig *cfg)
 {
-    int i;
-
    *cfg = ts->cfg;
-
-    for (i = 0; i < BUCKETS_COUNT; i++) {
-        throttle_unfix_bucket(&cfg->buckets[i]);
-    }
 }
 
 

Signature · 1 line

2.11.0

util/throttle.c static int64_t throttle_do_compute_wait(double limit, double extra) · line 110

Quoted 8 lines

util/throttle.c +6 -2

-    /* If the bucket is full then we have to wait */
-    extra = bkt->level - bkt->max * bkt->burst_length;
+    if (!bkt->max) {
+        /* If bkt->max is 0 we still want to allow short bursts of I/O
+         * from the guest, otherwise every other request will be throttled
+         * and performance will suffer considerably. */
+        bucket_size = bkt->avg / 10;
+        burst_bucket_size = 0;

Might be wrong, but shouldn't that be

    bucket_size = (bkt->avg / 10) * bkt->burst_length?
    burst_bucket_size = (bkt->avg / 10) / 10;

util/throttle.c static int64_t throttle_do_compute_wait(double limit, double extra) · line 119

Quoted 9 lines

util/throttle.c +9 -0

+    } else {
+        /* If we have a burst limit then we have to wait until all I/O
+         * at burst rate has finished before throttling to bkt->avg */
+        bucket_size = bkt->max * bkt->burst_length;
+        burst_bucket_size = bkt->max / 10;
+    }
+
+    /* If the main bucket is full then we have to wait */
+    extra = bkt->level - bucket_size;

because it used to be that if bkt->max is 0 then
extra = bkt->level - (bkt->avg /10) * bkt->burst_length; //fixed value and now it's
extra = bkt->level - (bkt->avg / 10);

and

util/throttle.c static int64_t throttle_do_compute_wait(double limit, double extra) · line 127

Quoted 13 lines

    if (extra > 0) {
        return throttle_do_compute_wait(bkt->avg, extra);
    }

util/throttle.c +3 -5

-    /* If the bucket is not full yet we have to make sure that we
-     * fulfill the goal of bkt->max units per second. */
+    /* If the main bucket is not full yet we still have to check the
+     * burst bucket in order to enforce the burst limit */
    if (bkt->burst_length > 1) {
-        /* We use 1/10 of the max value to smooth the throttling.
-         * See throttle_fix_bucket() for more details. */
-        extra = bkt->burst_level - bkt->max / 10;
+        extra = bkt->burst_level - burst_bucket_size;

This used to be

    extra = bkt->burst_level - (bkt->avg / 10) / 10;
and now it's
    extra = bkt->burst_level  - 0;

Quoted 9 lines · Manos Pitsidianakis

util/throttle.c static int64_t throttle_do_compute_wait(double limit, double extra) · line 110

Quoted 8 lines

util/throttle.c +6 -2

-    /* If the bucket is full then we have to wait */
-    extra = bkt->level - bkt->max * bkt->burst_length;
+    if (!bkt->max) {
+        /* If bkt->max is 0 we still want to allow short bursts of I/O
+         * from the guest, otherwise every other request will be throttled
+         * and performance will suffer considerably. */
+        bucket_size = bkt->avg / 10;
+        burst_bucket_size = 0;

Quoted 3 lines

Might be wrong, but shouldn't that be

    bucket_size = (bkt->avg / 10) * bkt->burst_length?
    burst_bucket_size = (bkt->avg / 10) / 10;

if !bkt->max then the user didn't define any bursts, and the I/O is only throttled to the rate set in bkt->avg.

Quoted 31 lines

util/throttle.c static int64_t throttle_do_compute_wait(double limit, double extra) · line 119

Quoted 9 lines

util/throttle.c +9 -0

+    } else {
+        /* If we have a burst limit then we have to wait until all I/O
+         * at burst rate has finished before throttling to bkt->avg */
+        bucket_size = bkt->max * bkt->burst_length;
+        burst_bucket_size = bkt->max / 10;
+    }
+
+    /* If the main bucket is full then we have to wait */
+    extra = bkt->level - bucket_size;

because it used to be that if bkt->max is 0 then
extra = bkt->level - (bkt->avg /10) * bkt->burst_length; //fixed value and now it's
extra = bkt->level - (bkt->avg / 10);

and

util/throttle.c static int64_t throttle_do_compute_wait(double limit, double extra) · line 127

Quoted 13 lines

    if (extra > 0) {
        return throttle_do_compute_wait(bkt->avg, extra);
    }

util/throttle.c +3 -5

-    /* If the bucket is not full yet we have to make sure that we
-     * fulfill the goal of bkt->max units per second. */
+    /* If the main bucket is not full yet we still have to check the
+     * burst bucket in order to enforce the burst limit */
    if (bkt->burst_length > 1) {
-        /* We use 1/10 of the max value to smooth the throttling.
-         * See throttle_fix_bucket() for more details. */
-        extra = bkt->burst_level - bkt->max / 10;
+        extra = bkt->burst_level - burst_bucket_size;

This used to be

Quoted 3 lines

    extra = bkt->burst_level - (bkt->avg / 10) / 10;
and now it's
    extra = bkt->burst_level  - 0;

No, if bkt->burst_length > 1 then bkt->max != 0, and therefore burst_bucket_size is never 0 either.

Perhaps you missed the ! in the first "if (!bkt->max)" ?

Both the throttling limits set with the throttling.iops-* and throttling.bps-* options and their QMP equivalents defined in the BlockIOThrottle struct are integer values.

Those limits are also reported in the BlockDeviceInfo struct and they are integers there as well.

Therefore there's no reason to store them internally as double and do the conversion everytime we're setting or querying them, so this patch uses int64_t for those types.

LeakyBucket.level and LeakyBucket.burst_level do however remain double because their value changes depending on the fraction of time elapsed since the previous I/O operation.

Signed-off-by Alberto Garcia <[email protected]>

include/qemu/throttle.h | 4 ++--
util/throttle.c | 4 ++--

2 files changed, 4 insertions(+), 4 deletions(-)

include/qemu/throttle.h +2 -2

index 66a8ac10a4..c466f6ccaa 100644
@@ -77,8 +77,8 @@ typedef enum {
 */
 
 typedef struct LeakyBucket {
-    double  avg;              /* average goal in units per second */
-    double  max;              /* leaky bucket max burst in units */
+    int64_t avg;              /* average goal in units per second */
+    int64_t max;              /* leaky bucket max burst in units */
    double  level;            /* bucket level in units */
    double  burst_level;      /* bucket level in units (for computing bursts) */
    unsigned burst_length;    /* max length of the burst period, in seconds */

util/throttle.c +2 -2

index 1f29cf9918..55a2451a14 100644
@@ -106,13 +106,13 @@ int64_t throttle_compute_wait(LeakyBucket *bkt)
        /* If bkt->max is 0 we still want to allow short bursts of I/O
         * from the guest, otherwise every other request will be throttled
         * and performance will suffer considerably. */
-        bucket_size = bkt->avg / 10;
+        bucket_size = (double) bkt->avg / 10;
        burst_bucket_size = 0;
    } else {
        /* If we have a burst limit then we have to wait until all I/O
         * at burst rate has finished before throttling to bkt->avg */
        bucket_size = bkt->max * bkt->burst_length;
-        burst_bucket_size = bkt->max / 10;
+        burst_bucket_size = (double) bkt->max / 10;
    }
 
    /* If the main bucket is full then we have to wait */

Signature · 1 line

2.11.0