From: Eric Wong <e@80x24.org>
To: spew@80x24.org
Subject: [PATCH 0/3] switch to tombstone-free khashl table
Date: Mon, 25 Mar 2024 23:03:35 +0000 [thread overview]
Message-ID: <20240325230339.262200-1-e@80x24.org> (raw)
The memory improvement is minor, but any memory reduction at all
is welcome at this point. Fortunately, this set of changes is
unintrusive.
I have some other ideas that I'll hopefully get to implement before
swapping kills all my SSDs (see bottom).
Eric Wong (3):
list-objects-filter: use kh_size API
treewide: switch to khashl for memory savings
khashl: fix ensemble lookups on empty table
builtin/fast-import.c | 2 +-
builtin/fsmonitor--daemon.c | 4 +-
delta-islands.c | 4 +-
khash.h | 338 -----------------------
khashl.h | 522 ++++++++++++++++++++++++++++++++++++
list-objects-filter.c | 2 +-
object-store-ll.h | 2 +-
object-store.h | 7 +-
oidset.h | 2 +-
pack-bitmap.h | 2 +-
10 files changed, 535 insertions(+), 350 deletions(-)
delete mode 100644 khash.h
create mode 100644 khashl.h
TODO (some ideas stolen from khashl):
* obj_hash can probably use a 0.75 load factor (instead of 0.5)
to save memory and not slow down too much. oid_* khash has
always had 0.77 which was fine and now khashl has 0.75.
0.75 is cheaper to compute via (shifts + ORs) than 0.77.
* obj_hash can use `i = (i + 1) & mask' like khashl does to
avoid branches in linear probe loops
* obj_hash can use realloc and copy in-place resize logic khashl does.
khashl uses the ->used bitmap for this, but it should be
possible to do for obj_hash w/o additional allocations
via pointer tagging
(not khashl inspired):
* keep an identity pool of OIDs separately (similar to how Perl5
pools all hash keys) and only use tagged pointers for OIDs.
Pointer tagging can be used to distinguish between 7 hash
functions, leaving us room for 5 more in addition to SHA-(1|256).
This change will be a large effort (with a hopefully large savings).
If we ever need more than 7 hash functions, we can switch to
storing hash type information in extents that can be looked up
using address ranges (AFAIK, jemalloc does this).
next reply other threads:[~2024-03-25 23:03 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-25 23:03 Eric Wong [this message]
2024-03-25 23:03 ` [PATCH 1/3] list-objects-filter: use kh_size API Eric Wong
2024-03-25 23:03 ` [PATCH 2/3] treewide: switch to khashl for memory savings Eric Wong
2024-03-25 23:03 ` [PATCH 3/3] khashl: fix ensemble lookups on empty table Eric Wong
2024-03-25 23:03 ` [REJECT 4/3] switch to khashl ensemble Eric Wong
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