mirror of
https://source.quilibrium.com/quilibrium/ceremonyclient.git
synced 2024-12-26 00:25:17 +00:00
93 lines
4.7 KiB
Markdown
93 lines
4.7 KiB
Markdown
# Memory Management
|
|
|
|
## Background
|
|
|
|
Pebble has two significant sources of memory usage: MemTables and the
|
|
Block Cache. MemTables buffer data that has been written to the WAL
|
|
but not yet flushed to an SSTable. The Block Cache provides a cache of
|
|
uncompressed SSTable data blocks.
|
|
|
|
Originally, Pebble used regular Go memory allocation for the memory
|
|
backing both MemTables and the Block Cache. This was problematic as it
|
|
put significant pressure on the Go GC. The higher the bandwidth of
|
|
memory allocations, the more work GC has to do to reclaim the
|
|
memory. In order to lessen the pressure on the Go GC, an "allocation
|
|
cache" was introduced to the Block Cache which allowed reusing the
|
|
memory backing cached blocks in most circumstances. This produced a
|
|
dramatic reduction in GC pressure and a measurable performance
|
|
improvement in CockroachDB workloads.
|
|
|
|
Unfortunately, the use of Go allocated memory still caused a
|
|
problem. CockroachDB running on top of Pebble often resulted in an RSS
|
|
(resident set size) 2x what it was when using RocksDB. The cause of
|
|
this effect is due to the Go runtime's heuristic for triggering GC:
|
|
|
|
> A collection is triggered when the ratio of freshly allocated data
|
|
> to live data remaining after the previous collection reaches this
|
|
> percentage.
|
|
|
|
This percentage can be configured by the
|
|
[`GOGC`](https://golang.org/pkg/runtime/) environment variable or by
|
|
calling
|
|
[`debug.SetGCPercent`](https://golang.org/pkg/runtime/debug/#SetGCPercent). The
|
|
default value is `100`, which means that GC is triggered when the
|
|
freshly allocated data is equal to the amount of live data at the end
|
|
of the last collection period. This generally works well in practice,
|
|
but the Pebble Block Cache is often configured to be 10s of gigabytes
|
|
in size. Waiting for 10s of gigabytes of data to be allocated before
|
|
triggering a GC results in very large Go heap sizes.
|
|
|
|
## Manual Memory Management
|
|
|
|
Attempting to adjust `GOGC` to account for the significant amount of
|
|
memory used by the Block Cache is fraught. What value should be used?
|
|
`10%`? `20%`? Should the setting be tuned dynamically? Rather than
|
|
introducing a heuristic which may have cascading effects on the
|
|
application using Pebble, we decided to move the Block Cache and
|
|
MemTable memory out of the Go heap. This is done by using the C memory
|
|
allocator, though it could also be done by providing a simple memory
|
|
allocator in Go which uses `mmap` to allocate memory.
|
|
|
|
In order to support manual memory management for the Block Cache and
|
|
MemTables, Pebble needs to precisely track their lifetime. This was
|
|
already being done for the MemTable in order to account for its memory
|
|
usage in metrics. It was mostly being done for the Block Cache. Values
|
|
stores in the Block Cache are reference counted and are returned to
|
|
the "alloc cache" when their reference count falls
|
|
to 0. Unfortunately, this tracking wasn't precise and there were
|
|
numerous cases where the cache values were being leaked. This was
|
|
acceptable in a world where the Go GC would clean up after us. It is
|
|
unacceptable if the leak becomes permanent.
|
|
|
|
## Leak Detection
|
|
|
|
In order to find all of the cache value leaks, Pebble has a leak
|
|
detection facility built on top of
|
|
[`runtime.SetFinalizer`](https://golang.org/pkg/runtime/#SetFinalizer). A
|
|
finalizer is a function associated with an object which is run when
|
|
the object is no longer reachable. On the surface, this sounds perfect
|
|
as a facility for performing all memory reclamation. Unfortunately,
|
|
finalizers are generally frowned upon by the Go implementors, and come
|
|
with very loose guarantees:
|
|
|
|
> The finalizer is scheduled to run at some arbitrary time after the
|
|
> program can no longer reach the object to which obj points. There is
|
|
> no guarantee that finalizers will run before a program exits, so
|
|
> typically they are useful only for releasing non-memory resources
|
|
> associated with an object during a long-running program
|
|
|
|
This language is somewhat frightening, but in practice finalizers are run at the
|
|
end of every GC period. Pebble primarily relies on finalizers for its leak
|
|
detection facility. In the block cache, a finalizer is associated with the Go
|
|
allocated `cache.Value` object. When the finalizer is run, it checks that the
|
|
buffer backing the `cache.Value` has been freed. This leak detection facility is
|
|
enabled by the `"invariants"` build tag which is enabled by the Pebble unit
|
|
tests.
|
|
|
|
There also exists a very specific memory reclamation use case in the block cache
|
|
that ensures that structs with transitively reachable fields backed by manually
|
|
allocated memory that are pooled in a `sync.Pool` are freed correctly when their
|
|
parent struct is released from the pool and consequently garbage collected by
|
|
the Go runtime (see `cache/entry_normal.go`). The loose guarantees provided by
|
|
the runtime are reasonable to rely on in this case to prevent a memory leak.
|