Appy created HBASE-18432:
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Summary: Prevent clock from getting stuck after update()
Key: HBASE-18432
URL: https://issues.apache.org/jira/browse/HBASE-18432
Project: HBase
Issue Type: Sub-task
Reporter: Appy
Assignee: Appy
There were a [bunch of
problems|https://issues.apache.org/jira/browse/HBASE-14070?focusedCommentId=16094013&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16094013]
(also copied below) with clock getting stuck after call to update() until it's
own system time caught up.
Proposed solution is, keeping track of skew separately.
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PT = physical time, LT = logical time, ST = system time, X = don't care terms
Note that in current implementation, we are passing master clock to RS in
open/close region request and RS clock to master in the responses. And they
both update their own time on receiving these request/response.
Also, on receiving a clock ahead of its own, they update their own clock to its
PT+LT, and keep increasing LT till their own ST catches that PT.
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Problem 1: Logical time window too small.
RS clock (10, X)
Master clock (20, X)
Master --request-> RS
RS clock (20, X)
While RS's physical java clock (which is backing up physical component of hlc
clock) will still take 10 sec to catch up, we'll keep incrementing logical
component. That means, in worst case, our logical clock window should be big
enough to support all the events that can happen in max skew time.
The problem is, that doesn't seem to be the case. Our logical window is 1M
events (20bits) and max skew time is 30 sec, that results in 33k max write qps,
which is quite low. We can easily see 150k update qps per beefy server with 1k
values.
Even 22 bits won't be enough. We'll need minimum of 23 bits and 20 sec max skew
time to support ~420k max events per second in worst case clock skew.
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Problem 2: Cascading logical time increment.
When more RS are involved say - 3 RS and 1 master. Let's say max skew is 30 sec.
HLC Clocks (physical time, logical time): X = don't care
RS1: (50, 100k)
Master: (40, X)
RS2: (30, X)
RS3: (20, X)
[RS3's ST behind RS1's by 30 sec.]
RS1 replies to master, sends it's clock (50,X).
Master's clock (50, X). It'll be another 10 sec before it's own physical clock
reaches 50, so HLC's PT will remain 50 for next 10 sec.
Master --> RS2
RS2's clock = (50, X).
RS2 keeps incrementing LT on writes (since it's own PT is behind) for few
seconds before it replies back to master with (50, X+ few 100k).
Master's clock = (50, X+ few 100k) [Since master's physical clock hasn't caught
up yet, note that it was 10 seconds behind, PT remains 50.].
Master --> RS3
RS3's clock (50, X+few 100k)
But RS3's ST is behind RS1's ST by 30 sec, which means it'll keep incrementing
LT for next 30 sec (unless it gets a newer clock from master).
But the problem is, RS3 has much smaller LT window than actual 1M!!
—
Problem 3: Single bad RS clock crashing the cluster:
If a single RS's clock is bad and a bit faster, it'll catch time and keep
pulling master's PT with it. If 'real time' is say 20, max skew time is 10, and
bad RS is at time 29.9, it'll pull master to 29.9 (via next response), and then
any RS less than 19.9, i.e. just 0.1 sec away from real time will die due to
higher than max skew.
This can bring whole clusters down!
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Problem 4: Time jumps (not a bug, but more of a nuisance)
Say a RS is behind master by 20 sec. On each communication from master, RS will
update its own PT to master's PT, and it'll remain that till RS's ST catches
up. If there are frequent communication from master, ST might never catch up
and RS's PT will actually look like discrete time jumps rather than continuous
time.
For eg. If master communicated with RS at times 30, 40, 50 (RSs corresponding
times are 10, 20, 30), than all events on RS between time [10, 50] will be
timestamped with either 30, 40 or 50.
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