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internal/cache: move the bulk of allocations off the Go heap #523

Merged
merged 6 commits into from
Feb 12, 2020
Merged

internal/cache: move the bulk of allocations off the Go heap #523

merged 6 commits into from
Feb 12, 2020

Commits on Feb 12, 2020

  1. internal/cache: move the bulk of allocations off the Go heap 

    Use C malloc/free for the bulk of cache allocations. This required
elevating `cache.Value` to a public citizen of the cache package. A
distinction is made between manually managed memory and automatically
managed memory. Weak handles can only be made from values stored in
automatically managed memory. Note that weak handles are only used for
the index, filter, and range-del blocks, so the number of weak handles
is O(num-tables).

A finalizer is set on `*allocCache` and `*Cache` in order to ensure that
any outstanding manually allocated memory is released when these objects
are collected.

When `invariants` are enabled, finalizers are also set on `*Value` and
sstable iterators to ensure that we're not leaking manually managed
memory.

Fixes #11
    @petermattis
    petermattis committed Feb 12, 2020
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  2. internal/cache: optimize allocation of manual Values 

    For manual Values, allocate the `Value` at the start of the backing
buffer. For large block caches, the number of Values can reach 500k-1m
or higher, and consume a significant fraction of the total allocated
objects in the Go heap.
    @petermattis
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  3. internal/cache: optimize allocation of entry objects 

    Each entry in the cache has an associated `entry` struct. For large
block caches, the number of entries can reach 500k-1m or higher, and
consume a significant fraction of the total allocated objects in the Go
heap. For manually managed values, the lifetime of the associated
`entry` is known and the `*entry` never escapes outside of the cache
package. This allows us to manually manage the memory for these entries,
which are the majority of entries in the cache.
    @petermattis
    petermattis committed Feb 12, 2020
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  4. ci: add no-invariants test configuration 

    The "invariants" build tag causes different code paths to be
utilized. Add a no-invariants test configuration for testing the
invariant-less code paths.

Change the race builds to not specify the "invariants" tags as doing so
causes them to frequently flake on the CI machines. This appears to be
due to the size of the machines (RAM available) and not something more
serious.
    @petermattis
    petermattis committed Feb 12, 2020
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  5. docs: add doc describing manual memory management

    @petermattis
    petermattis committed Feb 12, 2020
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  6. internal/cache: add robinHoodMap 

    Add `robinHoodMap` which is a hash map implemented using Robin Hood
hashing. `robinHoodMap` is specialized to use `key` and `*entry` as the
key and value for the map respectively. The memory for the backing array
is manually allocated. This lowers GC pressure by moving the allocated
memory out of the Go heap, and by moving all of `*entry` pointers out of
the Go heap.

Use `robinHoodMap` for both the `shard.blocks` and `shard.files` maps.

Old is Go-map, new is RobinHood-map.

name              old time/op  new time/op  delta
MapInsert-16       144ns ± 5%   101ns ± 8%  -29.88%  (p=0.000 n=10+9)
MapLookupHit-16    137ns ± 1%    47ns ± 1%  -65.58%  (p=0.000 n=8+10)
MapLookupMiss-16  89.9ns ± 4%  48.6ns ± 3%  -45.95%  (p=0.000 n=10+9)
    @petermattis
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