4e2f62143b
Commit 14bf09f447 added a single-chunk
layout to `managed_bytes`, which makes the overhead of `managed_bytes`
smaller in the common case of a small buffer.
But there was a bug in it. In the copy constructor of `managed_bytes`,
a copy of a single-chunk `managed_bytes` is made single-chunk too.
But this is wrong, because the source of the copy and the target
of the copy might have different preferred max contiguous allocation
sizes.
In particular, if a `managed_bytes` of size between 13 kiB and 128 kiB
is copied from the standard allocator into LSA, the resulting
`managed_bytes` is a single chunk which violates LSA's preferred
allocation size. (And therefore is placed by LSA in the standard
allocator).
In other words, since Scylla 6.0, cache and memtable cells
between 13 kiB and 128 kiB are getting allocated in the standard allocator
rather than inside LSA segments.
Consequences of the bug:
1. Effective memory consumption of an affected cell is rounded up to the nearest
power of 2.
2. With a pathological-enough allocation pattern
(for example, one which somehow ends up placing a single 16 kiB
memtable-owned allocation in every aligned 128 kiB span),
memtable flushing could theoretically deadlock,
because the allocator might be too fragmented to let the memtable
grow by another 128 kiB segment, while keeping the sum of all
allocations small enough to avoid triggering a flush.
(Such an allocation pattern probably wouldn't happen in practice though).
3. It triggers a bug in reclaim which results in spurious
allocation failures despite ample evictable memory.
There is a path in the reclaimer procedure where we check whether
reclamation succeeded by checking that the number of free LSA
segments grew.
But in the presence of evictable non-LSA allocations, this is wrong
because the reclaim might have met its target by evicting the non-LSA
allocations, in which case memory is returned directly to the
standard allocator, rather than to the pool of free segments.
If that happens, the reclaimer wrongly returns `reclaimed_nothing`
to Seastar, which fails the allocation.
Refs (possibly fixes) https://github.com/scylladb/scylladb/issues/21072
Fixes https://github.com/scylladb/scylladb/issues/22941
Fixes https://github.com/scylladb/scylladb/issues/22389
Fixes https://github.com/scylladb/scylladb/issues/23781
Scylla
What is Scylla?
Scylla is the real-time big data database that is API-compatible with Apache Cassandra and Amazon DynamoDB. Scylla embraces a shared-nothing approach that increases throughput and storage capacity to realize order-of-magnitude performance improvements and reduce hardware costs.
For more information, please see the ScyllaDB web site.
Build Prerequisites
Scylla is fairly fussy about its build environment, requiring very recent versions of the C++23 compiler and of many libraries to build. The document HACKING.md includes detailed information on building and developing Scylla, but to get Scylla building quickly on (almost) any build machine, Scylla offers a frozen toolchain, This is a pre-configured Docker image which includes recent versions of all the required compilers, libraries and build tools. Using the frozen toolchain allows you to avoid changing anything in your build machine to meet Scylla's requirements - you just need to meet the frozen toolchain's prerequisites (mostly, Docker or Podman being available).
Building Scylla
Building Scylla with the frozen toolchain dbuild is as easy as:
$ git submodule update --init --force --recursive
$ ./tools/toolchain/dbuild ./configure.py
$ ./tools/toolchain/dbuild ninja build/release/scylla
For further information, please see:
- Developer documentation for more information on building Scylla.
- Build documentation on how to build Scylla binaries, tests, and packages.
- Docker image build documentation for information on how to build Docker images.
Running Scylla
To start Scylla server, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --workdir tmp --smp 1 --developer-mode 1
This will start a Scylla node with one CPU core allocated to it and data files stored in the tmp directory.
The --developer-mode is needed to disable the various checks Scylla performs at startup to ensure the machine is configured for maximum performance (not relevant on development workstations).
Please note that you need to run Scylla with dbuild if you built it with the frozen toolchain.
For more run options, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --help
Testing
See test.py manual.
Scylla APIs and compatibility
By default, Scylla is compatible with Apache Cassandra and its API - CQL. There is also support for the API of Amazon DynamoDB™, which needs to be enabled and configured in order to be used. For more information on how to enable the DynamoDB™ API in Scylla, and the current compatibility of this feature as well as Scylla-specific extensions, see Alternator and Getting started with Alternator.
Documentation
Documentation can be found here. Seastar documentation can be found here. User documentation can be found here.
Training
Training material and online courses can be found at Scylla University. The courses are free, self-paced and include hands-on examples. They cover a variety of topics including Scylla data modeling, administration, architecture, basic NoSQL concepts, using drivers for application development, Scylla setup, failover, compactions, multi-datacenters and how Scylla integrates with third-party applications.
Contributing to Scylla
If you want to report a bug or submit a pull request or a patch, please read the contribution guidelines.
If you are a developer working on Scylla, please read the developer guidelines.
Contact
- The community forum and Slack channel are for users to discuss configuration, management, and operations of ScyllaDB.
- The developers mailing list is for developers and people interested in following the development of ScyllaDB to discuss technical topics.