The more we add elements to an HyperLogLog counter, the smaller is
the probability that we actually update some register.
From this observation it is easy to see how it is possible to use
caching of a previously computed cardinality and reuse it to serve
HLLCOUNT queries as long as no register was updated in the data
structure.
This commit does exactly this by using just additional 8 bytes for the
data structure to store a 64 bit unsigned integer value cached
cardinality. When the most significant bit of the 64 bit integer is set,
it means that the value computed is no longer usable since at least a
single register was modified and we need to recompute it at the next
call of HLLCOUNT.
The value is always stored in little endian format regardless of the
actual CPU endianess.
All the Redis functions that need to modify the string value of a key in
a destructive way (APPEND, SETBIT, SETRANGE, ...) require to make the
object unshared (if refcount > 1) and encoded in raw format (if encoding
is not already REDIS_ENCODING_RAW).
This was cut & pasted many times in multiple places of the code. This
commit puts the small logic needed into a function called
dbUnshareStringValue().
We need to be sure that you can save a dataset in a Redis instance,
reload it in a different architecture, and continue to count in the same
HyperLogLog structure.
So 32 and 64 bit, little or bit endian, must all guarantee to output the
same hash for the same element.
The new representation is more obvious, starting from the LSB of the
first byte and using bits going to MSB, and passing to next byte as
needed.
There was also a subtle error: first two bits were unused, everything
was carried over on the right of two bits, even if it worked because of
the code requirement of always having a byte more at the end.
During the rewrite the code was made safer trying to avoid undefined
behavior due to shifting an uint8_t for more than 8 bits.
To test the bitfield array of counters set/get macros from the Redis Tcl
suite is hard, so a specialized command that is able to test the
internals was developed.
This is not really an error but something that always happens for
example when creating a new cluster, or if the sysadmin rejoins manually
a node that is already known.
Since useless logs don't help, moved to VERBOSE level.
New config epochs must always be obtained incrementing the currentEpoch,
that is itself guaranteed to be >= the max configEpoch currently known
to the node.
redis-trib used to allocate slots not considering fractions of nodes
when computing the slots_per_node amount. So the fractional part was
carried over till the end of the allocation, where the last node
received a few more slots than any other (or a lot more if the cluster
was composed of many nodes).
The computation was changed to allocate slots more evenly when they are
not exactly divisible for the number of masters we have.
The slave election in Redis Cluster guarantees that slaves promoted to
masters always end with unique config epochs, however failures during
manual reshardings, software bugs and operational errors may in theory
cause two nodes to have the same configEpoch.
This commit introduces a mechanism to eventually always end with different
configEpochs if a collision ever happens.
As a (wanted) side effect, this also ensures that after a new cluster
is created, all nodes will end with a different configEpoch automatically.
Bug found by the continuous integration test running the Redis
with valgrind:
==6245== Invalid read of size 8
==6245== at 0x4C2DEEF: memcpy@GLIBC_2.2.5 (mc_replace_strmem.c:876)
==6245== by 0x41F9E6: freeMemoryIfNeeded (redis.c:3010)
==6245== by 0x41D2CC: processCommand (redis.c:2069)
memmove() size argument was accounting for an extra element, going
outside the bounds of the array.
In this commit:
* Decrement steps are semantically differentiated from the reserved FDs.
Previously both values were 32 but the meaning was different.
* Make it clear that we save setrlimit errno.
* Don't explicitly handle wrapping of 'f', but prevent it from
happening.
* Add comments to make the function flow more readable.
This integrates PR #1630
In sentinelFlushConfig() fd could be undefined when the following if
statement was true:
if (rewrite_status == -1) goto werr;
This could cause random file descriptors to get closed.
Previously, the (!fp) would only catch lack of free space
under OS X. Linux waits to discover it can't write until
it actually writes contents to disk.
(fwrite() returns success even if the underlying file
has no free space to write into. All the errors
only show up at flush/sync/close time.)
Fixesantirez/redis#1604
Also update the original REDIS_EVENTLOOP_FDSET_INCR to
include REDIS_MIN_RESERVED_FDS. REDIS_EVENTLOOP_FDSET_INCR
exists to make sure more than (maxclients+RESERVED) entries
are allocated, but we can only guarantee that if we include
the current value of REDIS_MIN_RESERVED_FDS as a minimum
for the INCR size.
This got removed in 2e5c394 during a new feature addition.
The prior commit had "break if masters.length == masters_count"
but we are guaranteed to aready have that condition met since
otherwise we would haven't gotten this far.
Without this break statement, it's possible some masters may
be forgotten and have zero replicas while other masters have
more than their requested number of replicas.
Thanks to carlos for pointing out this regression at:
https://groups.google.com/forum/#!topic/redis-db/_WVVqDw5B7c
This bug was introduced in 2e5c394f during a refactor.
It took me a while to understand what was going on with
the code, so I've refactored it further by:
- Replacing boolean values with meaningful symbols
- Replacing 'i' with a meaningful variable name
- Adding the proper abort check
- Factoring out now duplicated conditionals
- Adding optional verbose logging (we're inside *four*
different looping constructs, so it takes a while to
figure out where all the moving parts are)
- Updating comment for the section
This fixes a problem when the number of master instances
equaled the number of replica instances. Before, when
there were equal numbers of both, nodes_count would go to
zero, but the while loop would spin in i < @replicas because
i would never be updated (because the nodes_list of each ip
was length == 0, which triggered an endless loop of
next -> i = 0 -> 0 < 1? -> true -> next -> i = 0 ...)
Thanks to carlo who found this problem at:
https://groups.google.com/forum/#!topic/redis-db/_WVVqDw5B7c
Fun fact: rlim_t is an unsigned long long on all platforms.
Continually subtracting from a rlim_t makes it get smaller
and smaller until it wraps, then you're up to 2^64-1.
This was causing an infinite loop on Redis startup if
your ulimit was extremely (almost comically) low.
The case of (f > oldlimit) would never be met in a case like:
f = 150
while (f > 20) f -= 128
Since f is unsigned, it can't go negative and would
take on values of:
Iteration 1: 150 - 128 => 22
Iteration 2: 22 - 128 => 18446744073709551510
Iterations 3-∞: ...
To catch the wraparound, we use the previous value of f
stored in limit.rlimit_cur. If we subtract from f and
get a larger number than the value it had previously,
we print an error and exit since we don't have enough
file descriptors to help the user at this point.
Thanks to @bs3g for the inspiration to fix this problem.
Patches existed from @bs3g at antirez#1227, but I needed to repair a few other
parts of Redis simultaneously, so I didn't get a chance to use them.
The log messages about open file limits have always
been slightly opaque and confusing. Here's an attempt to
fix their wording, detail, and meaning. Users will have a
better understanding of how to fix very common problems
with these reworded messages.
Also, we handle a new error case when maxclients becomes less
than one, essentially rendering the server unusable. We
now exit on startup instead of leaving the user with a server
unable to handle any connections.
This fixes antirez#356 as well.
