Hi,
this patch reduces false positives in the detection of bfq_queues
associated with soft real-time applications. As such, this patch
reduces the damages caused by these false positives to other
applications. The patch proved to be very effective, as detailed in
the commit message. A sort of counterpart of this patch should follow
soon, this time for false positives in the detection of cooperating
processes (processes doing I/O close to each other).
I guess it is evidently too late for 4.15, so, Jens, please consider
this patch for 4.16. Of course if the patch turns out to be ok.
Thanks,
Paolo
Paolo Valente (1):
block, bfq: consider also past I/O in soft real-time detection
block/bfq-iosched.c | 115 ++++++++++++++++++++++++++++++++++++----------------
1 file changed, 81 insertions(+), 34 deletions(-)
--
2.10.0
BFQ privileges the I/O of soft real-time applications, such as video
players, to guarantee to these application a high bandwidth and a low
latency. In this respect, it is not easy to correctly detect when an
application is soft real-time. A particularly nasty false positive is
that of an I/O-bound application that occasionally happens to meet all
requirements to be deemed as soft real-time. After being detected as
soft real-time, such an application monopolizes the device. Fortunately,
BFQ will realize soon that the application is actually not soft
real-time and suspend every privilege. Yet, the application may happen
again to be wrongly detected as soft real-time, and so on.
As highlighted by our tests, this problem causes BFQ to occasionally
fail to guarantee a high responsiveness, in the presence of heavy
background I/O workloads. The reason is that the background workload
happens to be detected as soft real-time, more or less frequently,
during the execution of the interactive task under test. To give an
idea, because of this problem, Libreoffice Writer occasionally takes 8
seconds, instead of 3, to start up, if there are sequential reads and
writes in the background, on a Kingston SSDNow V300.
This commit addresses this issue by leveraging the following facts.
The reason why some applications are detected as soft real-time despite
all BFQ checks to avoid false positives, is simply that, during high
CPU or storage-device load, I/O-bound applications may happen to do
I/O slowly enough to meet all soft real-time requirements, and pass
all BFQ extra checks. Yet, this happens only for limited time periods:
slow-speed time intervals are usually interspersed between other time
intervals during which these applications do I/O at a very high speed.
To exploit these facts, this commit introduces a little change, in the
detection of soft real-time behavior, to systematically consider also
the recent past: the higher the speed was in the recent past, the
later next I/O should arrive for the application to be considered as
soft real-time. At the beginning of a slow-speed interval, the minimum
arrival time allowed for the next I/O usually happens to still be so
high, to fall *after* the end of the slow-speed period itself. As a
consequence, the application does not risk to be deemed as soft
real-time during the slow-speed interval. Then, during the next
high-speed interval, the application cannot, evidently, be deemed as
soft real-time (exactly because of its speed), and so on.
This extra filtering proved to be rather effective: in the above test,
the frequency of false positives became so low that the start-up time
was 3 seconds in all iterations (apart from occasional outliers,
caused by page-cache-management issues, which are out of the scope of
this commit, and cannot be solved by an I/O scheduler).
Signed-off-by: Paolo Valente <[email protected]>
Signed-off-by: Angelo Ruocco <[email protected]>
---
block/bfq-iosched.c | 115 ++++++++++++++++++++++++++++++++++++----------------
1 file changed, 81 insertions(+), 34 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index bcb6d21..a9e06217 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2917,45 +2917,87 @@ static bool bfq_bfqq_is_slow(struct bfq_data *bfqd, struct bfq_queue *bfqq,
* whereas soft_rt_next_start is set to infinity for applications that do
* not.
*
- * Unfortunately, even a greedy application may happen to behave in an
- * isochronous way if the CPU load is high. In fact, the application may
- * stop issuing requests while the CPUs are busy serving other processes,
- * then restart, then stop again for a while, and so on. In addition, if
- * the disk achieves a low enough throughput with the request pattern
- * issued by the application (e.g., because the request pattern is random
- * and/or the device is slow), then the application may meet the above
- * bandwidth requirement too. To prevent such a greedy application to be
- * deemed as soft real-time, a further rule is used in the computation of
- * soft_rt_next_start: soft_rt_next_start must be higher than the current
- * time plus the maximum time for which the arrival of a request is waited
- * for when a sync queue becomes idle, namely bfqd->bfq_slice_idle.
- * This filters out greedy applications, as the latter issue instead their
- * next request as soon as possible after the last one has been completed
- * (in contrast, when a batch of requests is completed, a soft real-time
- * application spends some time processing data).
+ * Unfortunately, even a greedy (i.e., I/O-bound) application may
+ * happen to meet, occasionally or systematically, both the above
+ * bandwidth and isochrony requirements. This may happen at least in
+ * the following circumstances. First, if the CPU load is high. The
+ * application may stop issuing requests while the CPUs are busy
+ * serving other processes, then restart, then stop again for a while,
+ * and so on. The other circumstances are related to the storage
+ * device: the storage device is highly loaded or reaches a low-enough
+ * throughput with the I/O of the application (e.g., because the I/O
+ * is random and/or the device is slow). In all these cases, the
+ * I/O of the application may be simply slowed down enough to meet
+ * the bandwidth and isochrony requirements. To reduce the probability
+ * that greedy applications are deemed as soft real-time in these
+ * corner cases, a further rule is used in the computation of
+ * soft_rt_next_start: the return value of this function is forced to
+ * be higher than the maximum between the following two quantities.
*
- * Unfortunately, the last filter may easily generate false positives if
- * only bfqd->bfq_slice_idle is used as a reference time interval and one
- * or both the following cases occur:
- * 1) HZ is so low that the duration of a jiffy is comparable to or higher
- * than bfqd->bfq_slice_idle. This happens, e.g., on slow devices with
- * HZ=100.
+ * (a) Current time plus: (1) the maximum time for which the arrival
+ * of a request is waited for when a sync queue becomes idle,
+ * namely bfqd->bfq_slice_idle, and (2) a few extra jiffies. We
+ * postpone for a moment the reason for adding a few extra
+ * jiffies; we get back to it after next item (b). Lower-bounding
+ * the return value of this function with the current time plus
+ * bfqd->bfq_slice_idle tends to filter out greedy applications,
+ * because the latter issue their next request as soon as possible
+ * after the last one has been completed. In contrast, a soft
+ * real-time application spends some time processing data, after a
+ * batch of its requests has been completed.
+ *
+ * (b) Current value of bfqq->soft_rt_next_start. As pointed out
+ * above, greedy applications may happen to meet both the
+ * bandwidth and isochrony requirements under heavy CPU or
+ * storage-device load. In more detail, in these scenarios, these
+ * applications happen, only for limited time periods, to do I/O
+ * slowly enough to meet all the requirements described so far,
+ * including the filtering in above item (a). These slow-speed
+ * time intervals are usually interspersed between other time
+ * intervals during which these applications do I/O at a very high
+ * speed. Fortunately, exactly because of the high speed of the
+ * I/O in the high-speed intervals, the values returned by this
+ * function happen to be so high, near the end of any such
+ * high-speed interval, to be likely to fall *after* the end of
+ * the low-speed time interval that follows. These high values are
+ * stored in bfqq->soft_rt_next_start after each invocation of
+ * this function. As a consequence, if the last value of
+ * bfqq->soft_rt_next_start is constantly used to lower-bound the
+ * next value that this function may return, then, from the very
+ * beginning of a low-speed interval, bfqq->soft_rt_next_start is
+ * likely to be constantly kept so high that any I/O request
+ * issued during the low-speed interval is considered as arriving
+ * to soon for the application to be deemed as soft
+ * real-time. Then, in the high-speed interval that follows, the
+ * application will not be deemed as soft real-time, just because
+ * it will do I/O at a high speed. And so on.
+ *
+ * Getting back to the filtering in item (a), in the following two
+ * cases this filtering might be easily passed by a greedy
+ * application, if the reference quantity was just
+ * bfqd->bfq_slice_idle:
+ * 1) HZ is so low that the duration of a jiffy is comparable to or
+ * higher than bfqd->bfq_slice_idle. This happens, e.g., on slow
+ * devices with HZ=100. The time granularity may be so coarse
+ * that the approximation, in jiffies, of bfqd->bfq_slice_idle
+ * is rather lower than the exact value.
* 2) jiffies, instead of increasing at a constant rate, may stop increasing
* for a while, then suddenly 'jump' by several units to recover the lost
* increments. This seems to happen, e.g., inside virtual machines.
- * To address this issue, we do not use as a reference time interval just
- * bfqd->bfq_slice_idle, but bfqd->bfq_slice_idle plus a few jiffies. In
- * particular we add the minimum number of jiffies for which the filter
- * seems to be quite precise also in embedded systems and KVM/QEMU virtual
- * machines.
+ * To address this issue, in the filtering in (a) we do not use as a
+ * reference time interval just bfqd->bfq_slice_idle, but
+ * bfqd->bfq_slice_idle plus a few jiffies. In particular, we add the
+ * minimum number of jiffies for which the filter seems to be quite
+ * precise also in embedded systems and KVM/QEMU virtual machines.
*/
static unsigned long bfq_bfqq_softrt_next_start(struct bfq_data *bfqd,
struct bfq_queue *bfqq)
{
- return max(bfqq->last_idle_bklogged +
- HZ * bfqq->service_from_backlogged /
- bfqd->bfq_wr_max_softrt_rate,
- jiffies + nsecs_to_jiffies(bfqq->bfqd->bfq_slice_idle) + 4);
+ return max3(bfqq->soft_rt_next_start,
+ bfqq->last_idle_bklogged +
+ HZ * bfqq->service_from_backlogged /
+ bfqd->bfq_wr_max_softrt_rate,
+ jiffies + nsecs_to_jiffies(bfqq->bfqd->bfq_slice_idle) + 4);
}
/**
@@ -4002,10 +4044,15 @@ static void bfq_init_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bfqq->split_time = bfq_smallest_from_now();
/*
- * Set to the value for which bfqq will not be deemed as
- * soft rt when it becomes backlogged.
+ * To not forget the possibly high bandwidth consumed by a
+ * process/queue in the recent past,
+ * bfq_bfqq_softrt_next_start() returns a value at least equal
+ * to the current value of bfqq->soft_rt_next_start (see
+ * comments on bfq_bfqq_softrt_next_start). Set
+ * soft_rt_next_start to now, to mean that bfqq has consumed
+ * no bandwidth so far.
*/
- bfqq->soft_rt_next_start = bfq_greatest_from_now();
+ bfqq->soft_rt_next_start = jiffies;
/* first request is almost certainly seeky */
bfqq->seek_history = 1;
--
2.10.0
On 12/14/17 11:23 PM, Paolo Valente wrote:
> Hi,
> this patch reduces false positives in the detection of bfq_queues
> associated with soft real-time applications. As such, this patch
> reduces the damages caused by these false positives to other
> applications. The patch proved to be very effective, as detailed in
> the commit message. A sort of counterpart of this patch should follow
> soon, this time for false positives in the detection of cooperating
> processes (processes doing I/O close to each other).
>
> I guess it is evidently too late for 4.15, so, Jens, please consider
> this patch for 4.16. Of course if the patch turns out to be ok.
Added for 4.16, thanks.
--
Jens Axboe