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virtio-devices: block: Fix latency counter for average read/write

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The cumulative average formula [1] requires to use signed integers
for proper calculations, while calculated result (e.g. cumulative
average) is always positive. This patch reflects the above requirements
in our code.

[1] https://en.wikipedia.org/wiki/Moving_average#Cumulative_average

Fixes: #5745

Signed-off-by: Bo Chen <chen.bo@intel.com>
This commit is contained in:
Bo Chen 2023-09-11 12:18:40 -07:00 committed by Rob Bradford
parent e151483f70
commit b76d0e8b50
1 changed files with 15 additions and 7 deletions
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22
virtio-devices/src/block.rs
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@ -309,9 +309,13 @@ impl BlockEpollHandler {
read_avg = if read_avg == u64::MAX { read_avg = if read_avg == u64::MAX {
latency * LATENCY_SCALE latency * LATENCY_SCALE
} else { } else {
read_avg // Cumulative average is guaranteed to be
+ ((latency * LATENCY_SCALE) - read_avg) // positive if being calculated properly
/ (read_ops_last + read_ops.0) (read_avg as i64
+ ((latency * LATENCY_SCALE) as i64 - read_avg as i64)
/ (read_ops_last + read_ops.0) as i64)
.try_into()
.unwrap()
}; };
} }
RequestType::Out => { RequestType::Out => {
@ -339,10 +343,14 @@ impl BlockEpollHandler {
write_avg = if write_avg == u64::MAX { write_avg = if write_avg == u64::MAX {
latency * LATENCY_SCALE latency * LATENCY_SCALE
} else { } else {
write_avg // Cumulative average is guaranteed to be
+ ((latency * LATENCY_SCALE) - write_avg) // positive if being calculated properly
/ (write_ops_last + write_ops.0) (write_avg as i64
}; + ((latency * LATENCY_SCALE) as i64 - write_avg as i64)
/ (write_ops_last + write_ops.0) as i64)
.try_into()
.unwrap()
}
} }
_ => {} _ => {}
} }