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erlfdb_hca.erl
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erlfdb_hca.erl
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% Licensed under the Apache License, Version 2.0 (the "License"); you may not
% use this file except in compliance with the License. You may obtain a copy of
% the License at
%
% http://www.apache.org/licenses/LICENSE-2.0
%
% Unless required by applicable law or agreed to in writing, software
% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
% License for the specific language governing permissions and limitations under
% the License.
-module(erlfdb_hca).
% This is based on the FoundationDB Python bindings as well
% as this excellent blog post describing the logic:
% https://ananthakumaran.in/2018/08/05/high-contention-allocator.html
-export([
create/1,
allocate/2
]).
-include("erlfdb.hrl").
-record(erlfdb_hca, {
counters,
recent
}).
create(Prefix) ->
#erlfdb_hca{
counters = ?ERLFDB_EXTEND(Prefix, 0),
recent = ?ERLFDB_EXTEND(Prefix, 1)
}.
allocate(HCA, Db) ->
try
erlfdb:transactional(Db, fun(Tx) ->
Start = current_start(HCA, Tx),
CandidateRange = range(HCA, Tx, Start, false),
search_candidate(HCA, Tx, CandidateRange)
end)
catch throw:hca_retry ->
allocate(HCA, Db)
end.
current_start(HCA, Tx) ->
#erlfdb_hca{
counters = Counters
} = HCA,
{CRangeStart, CRangeEnd} = ?ERLFDB_RANGE(Counters),
Options = [
{snapshot, true},
{reverse, true},
{streaming_mode, exact},
{limit, 1}
],
case erlfdb:wait(erlfdb:get_range(Tx, CRangeStart, CRangeEnd, Options)) of
[] ->
0;
[{CounterKey, _}] ->
{Start} = ?ERLFDB_EXTRACT(Counters, CounterKey),
Start
end.
range(HCA, Tx, Start, WindowAdvanced) ->
#erlfdb_hca{
counters = Counters
} = HCA,
if not WindowAdvanced -> ok; true ->
clear_previous_window(HCA, Tx, Start)
end,
CounterKey = ?ERLFDB_EXTEND(Counters, Start),
erlfbd:add(Tx, CounterKey, 1),
Count = case erlfdb:wait(erlfdb:get_ss(Tx, CounterKey)) of
<<C:64/little>> -> C;
not_found -> 0
end,
WindowSize = window_size(Start),
case Count * 2 < WindowSize of
true -> {Start, WindowSize};
false -> range(HCA, Tx, Start + WindowSize, true)
end.
search_candidate(HCA, Tx, {Start, WindowSize}) ->
#erlfdb_hca{
counters = Counters,
recent = Recent
} = HCA,
% -1 because random:uniform is 1 =< X $=< WindowSize
Candidate = Start + rand:uniform(WindowSize) - 1,
CandidateValueKey = ?ERLFDB_EXTEND(Recent, Candidate),
Options = [
{snapshot, true},
{reverse, true},
{streaming_mode, exact},
{limit, 1}
],
{CRangeStart, CRangeEnd} = ?ERLFDB_RANGE(Counters),
CFuture = erlfdb:get_range(Tx, CRangeStart, CRangeEnd, Options),
CVFuture = erlfdb:get(Tx, CandidateValueKey),
erlfdb:set_option(Tx, next_write_no_write_conflict_range),
erlfdb:set(Tx, CandidateValueKey, <<>>),
[LatestCounter, CandidateValue] = erlfdb:wait_for_all([CFuture, CVFuture]),
% Check that we're still in the same counter window
case LatestCounter of
[{CounterKey, _}] ->
{LStart} = ?ERLFDB_EXTRACT(Counters, CounterKey),
if LStart == Start -> ok; true ->
throw(hca_restart)
end;
_ ->
ok
end,
case CandidateValue of
not_found ->
erlfdb:add_write_conflict_key(Tx, CandidateValueKey),
erlfdb_tuple:pack({Candidate});
_ ->
throw(hca_restart)
end.
clear_previous_window(HCA, Tx, Start) ->
#erlfdb_hca{
counters = Counters,
recent = Recent
} = HCA,
{CRangeStart, CRangeEnd} = ?ERLFDB_RANGE(Counters, Start),
{RRangeStart, RRangeEnd} = ?ERLFDB_RANGE(Recent, Start),
erlfdb:clear_range(Tx, CRangeStart, CRangeEnd),
erlfdb:set_option(Tx, next_write_no_conflict_range),
erlfdb:clear_range(Tx, RRangeStart, RRangeEnd).
window_size(Start) ->
if
Start < 255 -> 64;
Start < 65535 -> 1024;
true -> 8192
end.