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Reduce memory & redundant work for concurrent TimeZones construction #356

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5bffed5
Reduce memory & redundant work for concurrent TimeZones construction
NHDaly Aug 19, 2021
6c0d3ea
Add `@lock` macro on julia 1.0
NHDaly Aug 20, 2021
beacaaa
PR Feedback from review.
NHDaly Jan 2, 2022
96252ab
Merge branch 'master' into nhd/share-cache
NHDaly Jan 2, 2022
4f444eb
Drop compat for `Base.@lock` now that we're on julia 1.6+ as LTS :)
NHDaly Jan 2, 2022
0258929
PR feedback
NHDaly Feb 13, 2022
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4 changes: 3 additions & 1 deletion src/TimeZones.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,8 @@ using RecipesBase: RecipesBase, @recipe
using Unicode
using InlineStrings: InlineString15

using Base: @lock

import Dates: TimeZone, UTC

export TimeZone, @tz_str, istimezone, FixedTimeZone, VariableTimeZone, ZonedDateTime,
Expand Down Expand Up @@ -40,7 +42,7 @@ abstract type Local <: TimeZone end

function __init__()
# Initialize the thread-local TimeZone cache (issue #342)
_reset_tz_cache()
_init_tz_cache()

# Base extension needs to happen everytime the module is loaded (issue #24)
Dates.CONVERSION_SPECIFIERS['z'] = TimeZone
Expand Down
68 changes: 53 additions & 15 deletions src/types/timezone.jl
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Expand Up @@ -4,6 +4,12 @@
# to the cache, while still being thread-safe.
const THREAD_TZ_CACHES = Vector{Dict{String,Tuple{TimeZone,Class}}}()

# Holding a lock during construction of a specific TimeZone prevents multiple Tasks (on the
# same or different threads) from attempting to construct the same TimeZone object, and
# allows them all to share the result.
const TZ_CACHE_MUTEX = ReentrantLock()
const TZ_CACHE_FUTURES = Dict{String,Channel{Tuple{TimeZone,Class}}}() # Guarded by: TZ_CACHE_MUTEX

# Based upon the thread-safe Global RNG implementation in the Random stdlib:
# https://github.com/JuliaLang/julia/blob/e4fcdf5b04fd9751ce48b0afc700330475b42443/stdlib/Random/src/RNGs.jl#L369-L385
@inline _tz_cache() = _tz_cache(Threads.threadid())
Expand All @@ -19,10 +25,22 @@ const THREAD_TZ_CACHES = Vector{Dict{String,Tuple{TimeZone,Class}}}()
end
@noinline _tz_cache_length_assert() = @assert false "0 < tid <= length(THREAD_TZ_CACHES)"

function _reset_tz_cache()
# ensures that we didn't save a bad object
function _init_tz_cache()
resize!(empty!(THREAD_TZ_CACHES), Threads.nthreads())
end
# ensures that we didn't save a bad object
function _reset_tz_cache()
# Since we use thread-local caches, we spawn a task on _each thread_ to clear that
# thread's local cache.
Threads.@threads for i in 1:Threads.nthreads()
@assert Threads.threadid() === i "TimeZones.TZData.compile() must be called from the main, top-level Task."
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How is this guaranteed when calling compile/_reset_tz_cache from the main task?

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I don't remember / understand the reason, but the Threads.@threads macro only works from the top-level task.

The behavior of @threads is that it evenly divides the for-loop across the number of threads, so if you have exactly nthreads() iterations, exactly one iteration will go on each thread.

help?> Threads.@threads
  Threads.@threads [schedule] for ... end

  A macro to parallelize a for loop to run with multiple threads. Splits the iteration space among multiple tasks and runs those tasks on threads according
  to a scheduling policy. A barrier is placed at the end of the loop which waits for all tasks to finish execution.

  The schedule argument can be used to request a particular scheduling policy. The only currently supported value is :static, which creates one task per
  thread and divides the iterations equally among them. Specifying :static is an error if used from inside another @threads loop or from a thread other than
  1.

  The default schedule (used when no schedule argument is present) is subject to change.

  │ Julia 1.5
  │
  │  The schedule argument is available as of Julia 1.5.

It only works from thread 1, for reasons i can't quite remember, but so this basically means you have to start it from the main Task. (I didn't remember that it was a "thread 1" requirement - i thought it was actually a "main Task" requirement.. i can consider changing the assertion message, perhaps? But i think it's easier guidance to say "don't call this concurrently, dude")

empty!(_tz_cache())
end
@lock TZ_CACHE_MUTEX begin
empty!(TZ_CACHE_FUTURES)
end
return nothing
end

"""
TimeZone(str::AbstractString) -> TimeZone
Expand Down Expand Up @@ -68,20 +86,40 @@ function TimeZone(str::AbstractString, mask::Class=Class(:DEFAULT))
# Note: If the class `mask` does not match the time zone we'll still load the
# information into the cache to ensure the result is consistent.
tz, class = get!(_tz_cache(), str) do
tz_path = joinpath(TZData.COMPILED_DIR, split(str, "/")...)

if isfile(tz_path)
open(deserialize, tz_path, "r")
elseif occursin(FIXED_TIME_ZONE_REGEX, str)
FixedTimeZone(str), Class(:FIXED)
elseif !isdir(TZData.COMPILED_DIR) || isempty(readdir(TZData.COMPILED_DIR))
# Note: Julia 1.0 supresses the build logs which can hide issues in time zone
# compliation which result in no tzdata time zones being available.
throw(ArgumentError(
"Unable to find time zone \"$str\". Try running `TimeZones.build()`."
))
# Even though we're using Thread-local caches, we still need to lock during
# construction to prevent multiple tasks redundantly constructing the same object,
# and potential thread safety violations due to Tasks migrating threads.
# NOTE that we only grab the lock if the TZ doesn't exist, so the mutex contention
# is not on the critical path for most constructors. :)
constructing = false
# We lock the mutex, but for only a short, *constant time* duration, to grab the
# future for this TimeZone, or create the future if it doesn't exist.
future = @lock TZ_CACHE_MUTEX begin
get!(TZ_CACHE_FUTURES, str) do
constructing = true
Channel{Tuple{TimeZone,Class}}(1)
end
end
if constructing
tz_path = joinpath(TZData.COMPILED_DIR, split(str, "/")...)

t = if isfile(tz_path)
open(deserialize, tz_path, "r")
elseif occursin(FIXED_TIME_ZONE_REGEX, str)
FixedTimeZone(str), Class(:FIXED)
elseif !isdir(TZData.COMPILED_DIR) || isempty(readdir(TZData.COMPILED_DIR))
# Note: Julia 1.0 supresses the build logs which can hide issues in time zone
# compliation which result in no tzdata time zones being available.
throw(ArgumentError(
"Unable to find time zone \"$str\". Try running `TimeZones.build()`."
))
else
throw(ArgumentError("Unknown time zone \"$str\""))
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Exceptions while constructing will cause threads to be blocked upon waiting for a channel that will never be populated

end

put!(future, t)
else
throw(ArgumentError("Unknown time zone \"$str\""))
fetch(future)
end
end

Expand Down
2 changes: 1 addition & 1 deletion src/tzdata/TZData.jl
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Expand Up @@ -2,7 +2,7 @@ module TZData

using LazyArtifacts
using Printf
using ...TimeZones: DEPS_DIR
using ...TimeZones: DEPS_DIR, _reset_tz_cache

# Note: The tz database is made up of two parts: code and data. TimeZones.jl only requires
# the "tzdata" archive or more specifically the "tz source" files within the archive
Expand Down
9 changes: 2 additions & 7 deletions src/tzdata/compile.jl
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Expand Up @@ -2,7 +2,7 @@ using Dates
using Serialization
using Dates: parse_components

using ...TimeZones: _tz_cache
using ...TimeZones: _reset_tz_cache
using ...TimeZones: TimeZones, TimeZone, FixedTimeZone, VariableTimeZone, Transition, Class
using ...TimeZones: rename
using ..TZData: TimeOffset, ZERO, MIN_GMT_OFFSET, MAX_GMT_OFFSET, MIN_SAVE, MAX_SAVE,
Expand Down Expand Up @@ -696,12 +696,7 @@ function compile(tz_source::TZSource, dest_dir::AbstractString; kwargs...)
isdir(dest_dir) || error("Destination directory doesn't exist")
# When we recompile the TimeZones from a new source, we clear all the existing cached
# TimeZone objects, so that newly constructed objects pick up the newly compiled rules.
# Since we use thread-local caches, we spawn a task on _each thread_ to clear that
# thread's local cache.
Threads.@threads for i in 1:Threads.nthreads()
@assert Threads.threadid() === i "TimeZones.TZData.compile() must be called from the main, top-level Task."
empty!(_tz_cache())
end
_reset_tz_cache()

for (tz, class) in results
parts = split(TimeZones.name(tz), '/')
Expand Down