Set up scheduling groups for streaming, compaction, memtable flush, query,
and commitlog.
The background writer scheduling group is retired; it is split into
the memtable flush and compaction groups.
Comments from Glauber:
This patch is based in a patch from Avi with the same subject, but the
differences are signficant enough so that I reset authorship. In
particular:
1) A bug/regression is fixed with the boundary calculations for the
memtable controller sampling function.
2) A leftover is removed, where after flushing a memtable we would
go back to the main group before going to the cache group again
3) As per Tomek's suggestion, now the submission of compactions
themselves are run in the compaction scheduling group. Having that
working is what changes this patch the most: we now store the
scheduling group in the compaction manager and let the compaction
manager itself enforce the scheduling group.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
thread_scheduling_groups are converted to plain scheduling_group. Due to
differences in initialization (scheduling_group initializtion defers), we
create the scheduling_groups in main.cc and propagate them to users via
a new class database_config.
The sstable writer loses its thread_scheduling_group parameter and instead
inherits scheduling from its caller.
Since shares are in the 1-1000 range vs. 0-1 for thread scheduling quotas,
the flush controller was adjusted to return values within the higher ranges.
"This patchset implements the compaction controller for I/O shares. The
goal is to automatic adjust compaction shares based on a
strategy-specific backlog. A higher backlog will translate into higher
shares.
As compaction progresses, that reduces the backlog. As new data is
flushed, that increases the backlog. The goal of the controler is to
keep the backlog constant at a certain rate, so that we don't go neither
too fast or too slow.
Tracking reads and writes:
==========================
Tracking of reads and writes happen through the read_monitor and the
write_monitor. The write monitor is an existing interface that has the
purpose of releasing the write permit at particular points of the write
process. We enhance it so to get a reference to an instance that tracks
the current offset inside the sstables::file_writer. This way the
backlog tracker can always know for sure what's the offset of the
current write.
A similar thing is done for reads. The data_consumer already tracks the
position of the current read, and we isolate that into a structure to
which we can get a reference. A read_monitor allows us to connect the
compaction to that reference.
Lifetime management:
====================
In general, tracking objects will be owned by their callers and passed
down as references. The compaction object will own the read monitors and
the compaction write monitors and the memtable flush write monitor will
be kept alive in a do_with block around the flush itself.
The backlog_{write,read}_progress_manager needs to be kept alive until
the SSTable is no longer in progress. For writes, that means until we
are able to add the SSTable charges in full, and for reads (compaction)
that means until we are able to remove the charges in full.
It is important to do that to avoid spikes in the graph. If we remove
the progress managers in a different operation than updating the SSTable
list we will be left in a temporary state where charges appear or
disappear abruptly, to be fixed when the final
add_sstable/remove_sstable happens. So we want those things to happen
together.
The compaction_backlog_tracker is kept alive until the strategy changes,
for example, through ALTER TABLE. Current charges are transferred to the
new strategy's compaction_backlog_tracker object when we do that. If the
type of strategy changes, the current read charges are forgotten. We can
do that because those running compaction will not really contribute to
decrease the backlog of the new compaction strategy.
Tranfer of Charges
==================
When ALTER TABLE happens, we need to transfer ongoing writes to the new
backlog manager. Ongoing reads will still be tracked by the
backlog_manager that originated them.
The rationale for that is that reads still belong to the current
compaction, with the strategy that generated them. But new Tables being
written will add to the backlog of the new strategy.
Note that ALTER TABLE operations not necessarily cause a change of
Strategy. We can be using the same strategy but just changing
properties. If that is the case, we expect no discontinuity in the
backlog graph (tested).
Resharding
==========
Resharding compactions are more complex than normal compactions because
the SSTables are created in one shard and later sent to another shard.
It is better, then, to track resharding compactions separately and let
them have their own backlog tracker, which will insert backlog in
proportion to the amount of data to be resharded.
Memtable Flush I/O Controller
=============================
With the current infrastructure it becomes trivial to add a new
controller, for either I/O or CPU. This patchset then adds an I/O
controller for memtable flushes, using the same backlog algorithm that
we already used for CPU."
* 'compaction-controller-io-v5' of github.com:glommer/scylla:
database: add a controller for I/O on memtable flushes.
document the compaction controller
compaction: adjust shares for compactions
backlog_controllers: implement generic I/O controller
factor out some of the controller code
io shares: multiply all shares by 10
compaction_strategy: implement backlog manager for the SizeTiered strategy
infrastructure for backlog estimator for compaction work.
sstables: notify about end of data component write
sstables: add read_monitor_generator
sstables: add read_monitor
sstables: enhance data consumer with a position tracker
sstables: enhance the file_writer with an offset tracker
sstables: pass references instead of pointers for write_monitor
compaction: control destruction of readers
This patch adds infrastucture in various points in the system to allow
us to determine the amount of work present as backlog from compactions.
What needs to be done can be explained in three major pieces:
1) Add hooks in the points where sstables are added or inserted to a
column family (or more precisely, to a compaction_strategy object).
2) Add hooks in reads and write monitors that allows a compaction
backlog estimator (tracker) to become aware of bytes that are
partially written and compacted away.
3) Add a per-column family class (compaction_backlog_tracker) that
can be used to track work that is done and relevant to compactions
(like the two above), and a compaction manager to provide a
system-wide backlog based on the response of the individual trackers.
The definition of how much backlog one has is strategy-specific. The
Null strategy is easy, as it never really has any backlog, and so is the
major strategy - since what it really matters is the backlog of the
underlying compaction strategy.
Although backlogs are strategy-specific, they should be "compatible", in
the sense that if a particular strategy has more work to do, it should
yield a higher number than its counterparts.
All the others are presented in this patch as unimplemented: they will
always advertise a mild backlog that should yield a constant
CPU-utilization if used alone.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
Currently, compaction manager will serialize compaction of same size tier
(or weight) if they belong to the same column family. However, it fails to
do so if the compaction jobs belong to different column families.
That can lead to an ungodly amount of running compaction which gets worse
the higher the number of shards and active column families. The problem
is that it may affect overall system performance due to excessive resource
usage. It's easy to trigger it during bootstraping after loading node with
new sstables or repairing, or if lots of cfs are being actively written.
That being said, compaction jobs of same size tier are now serialized
on a given shard, such that maximum number of compaction (system wise)
is now:
(SHARDS) * (SIZE TIERS)
instead of:
(SHARDS) * (COLUMN FAMILIES) * (SIZE TIERS)
We'll work hard to release a size tier (weight) for a column family
waiting on it as fast as possible, given that we wouldn't like to
underutilize resources available for compaction. We want one starting
after the other. Compaction for a column family that cannot run now
because the size tier is taken, will be postponed. There's a worker
that will be sleeping on a condition variable that will be signalled
whenever a compaction completes. FIFO ordering is used on postponed
list for fairness.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Motivation is that a new field in the descriptor will be forwarded
to compaction procedure without extending parameter list even more.
Also beautifies the interface, making it concise and easier to
play with.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
unordered_set will allow us to quickly extract fully expired tables
from a set of compacting sstables.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
The reason to do that is because compaction can deadlock if refresh
disables write which waits for compaction, and compaction in turn
waits for dirty memory[1] that would be released by memtable write.
Dirty memory manager for non-system cfs was being used for system cfs,
which was useful for exposing this problem.
[1]: when updating compaction history.
Fixes#2769.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <20170918215238.9810-2-raphaelsc@scylladb.com>
In scylla, we have foreground processes, which are latency sensitive and
need to be responded to as fast as possible in order to maintain good
latency profiles, and background process, which are less so.
The most important background processes we have during normal write
workload operations are memtable writes and sstable compactions. Those
processes are quite CPU-intensive, and left unchecked will easily
dominate the CPU. Lower values of task-quota usually help, as it will
force those processes to preempt more, but aren't enough to guarantee
good isolation. We have seen boxes with good NVMe storage having their
throughput reduced to less than half of the original baseline in a short
dive down for the duration of a compaction.
In the long run, our goal is to leverage the CPU scheduler to make sure
that those processes are balanced with respect to all the others.
However, the current state of affairs is causing grievances as this very
moment. Thankfully, those processes live in a seastar::thread, that
ships with its own rudimentary bandwidth control mechanism: the
scheduling group.
The goal of this patch is to wrap background processes together in a
scheduling group, and assign to such group 50 % of our CPU power; the
remainder being left to foreground processes.
While we pride ourselves in dynamically adjusting things to the
workload, we won't be able to do this properly before the CPU scheduler
lands - and let's face it, leaving background processes run wild is not
adaptative either. Every workload would benefit most from a different
value for such shares, but 50 % is as fair as it gets if we really need
static partitining in the mean time.
As a defense against unforeseen consequences, we'll leave the actual
value as an option, but will do our best to hide it - as this is not a
tunable that we want to be part of a normal Scylla setup. The most
convenient place for this tunable is still db::config, so we can easily
pass it down to the database layer - but we will not document it in the
yaml, and will clearly note in the help string that it is not supposed
to be tuned.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
two variants of size_tiered_most_interesting_bucket existed to avoid copy,
but subsequent work will make lcs use vector for each level of sstables,
so let's only keep one variant.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
so now user can look at nodetool compactionstats and determine
whether or not resharding is running, for example:
$ ./bin/nodetool compactionstats
pending tasks: 3
id compaction type keyspace table completed total unit progress
<none> RESHARD system compaction_history 11 256 keys 4.30%
<none> RESHARD system compaction_history 2 256 keys 0.78%
<none> RESHARD system compaction_history 10 256 keys 3.91%
<none> RESHARD system compaction_history 8 256 keys 3.12%
<none> RESHARD system compaction_history 7 256 keys 2.73%
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <20170620175733.25882-1-raphaelsc@scylladb.com>
After compaction revamp, compaction type set by cleanup at its ctor
is being overwritten at compaction::setup(). Consequently, cleanup
would not be stopped by 'nodetool stop cleanup' and cleanup would
be listed as regular compaction in 'nodetool compactionstats'.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <20170504013046.23522-1-raphaelsc@scylladb.com>
Strategies other than leveled will reshard one shared sstable at
a time, and the target shard, shard at which job will run, for each
job will be chosen in a round-robin fashion.
For leveled strategy, we will reshard together smp::count adjacent
sstables that belong to same level.
The reason for that is because resharding one sstable at a time
may result in creation of file for each shard, meaning after
resharding we could end up with NO_SSTABLES*NO_SHARDS.
These resharding strategies will be used for our new resharding
algorithm.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Extends compaction for new resharding algorithm. Not wired yet.
New resharding will compact shared sstable(s) and create one
sstable for each owner. It's up to the caller to open these
new unshared sstables at their respective column families.
This new approach will save a lot of bandwidth because we'll
no longer read the entire shared sstable #smp::count times.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Strongly based on org.apache.cassandra.db.compaction.
CompactionController.getFullyExpiredSSTables.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
When scylla stopped an ongoing compaction, the event was reported
as an error. This patch introduces a specialized exception for
compaction stop so that the event can be handled appropriately.
Before:
ERROR [shard 0] compaction_manager - compaction failed: read exception:
std::runtime_error (Compaction for keyspace1/standard1 was deliberately
stopped.)
After:
INFO [shard 0] compaction_manager - compaction info: Compaction for
keyspace1/standard1 was stopped due to shutdown.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <1f85d4e5c24d23a1b4e7e0370a2cffc97cbc6d44.1455034236.git.raphaelsc@scylladb.com>
That's needed for nodetool stop, which is called to stop all ongoing
compaction. The implementation is about informing an ongoing compaction
that it was asked to stop, so the compaction itself will trigger an
exception. Compaction manager will catch this exception and re-schedule
the compaction.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
compaction_info makes more sense because this structure doesn't
only store stats about ongoing compaction. Soon, we will add
information to it about whether or not an user asked to stop the
respective ongoing compaction.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Cleanup is about rewriting a sstable discarding any keys that
are irrelevant, i.e. keys that don't belong to current node.
Parameter cleanup was added to compact_sstables.
If set to true, irrelevant code such as the one that updates
compaction history will be skipped. Logic was also added to
discard irrelevant keys.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Adapt our compaction code to start writing a new sstable if the
one being written reached its maximum size. Leveled strategy works
with that concept. If a strategy other than leveled is being used,
everything will work as before.
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
That's helpful for the purpose of testing, and leveled compaction may
also end up using size-tiered compaction strategy for selecting
candidates.
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Instead of requiring the user to subclass a "sstable_creator" class to
specify how to create a new sstable (or in the future, several of them),
switch to an std::function.
In practice, it is much easier to specify a lambda than a class, especialy
since C++11 made it easy to capture variables into lambdas - but not into
local classes.
The "commit()" function is also unnecessary. Then intention there was to
provide a function to "commit" the new sstables (i.e., rename them).
But the caller doesn't need to supply this function - it can just wait
for the future of the end of compaction, and do his own committing code
right then.
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
This patch adds the basic compaction function sstables::compact_sstables,
which takes a list of input sstables, and creates several (currently one)
merged sstable. This implementation is pretty simple once we have all
the infrastructure in place (combining reader, writer, and a pipe between
them to reduce context switches).
This is already working compaction, but not quite complete: We'll need
to add compaction strategies (which sstables to compact, and when),
better cardinality estimator, sstable management and renaming, and a lot
of other details, and we'll probably still need to change the API.
But we can already write a test for compacting existing sstables (see
the next patch), and I wanted to get this patch out of the way, so we can
start working on applying compaction in a real use case.
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
