diff --git a/src/ThreadsX.jl b/src/ThreadsX.jl
index ded807ad..9a1c5c58 100644
--- a/src/ThreadsX.jl
+++ b/src/ThreadsX.jl
@@ -82,6 +82,7 @@ else
     foldxt(rf, xs; kw...) = reduce(rf, Map(identity), xs; kw...)
 end
 
+include("debug.jl")
 include("utils.jl")
 include("basesizes.jl")
 include("reduce.jl")
diff --git a/src/debug.jl b/src/debug.jl
new file mode 100644
index 00000000..1e0843ff
--- /dev/null
+++ b/src/debug.jl
@@ -0,0 +1,13 @@
+"""
+    @DBG expression_to_run_only_when_debugging
+"""
+macro DBG(ex)
+    quote
+        isdebugging() && $(esc(ex))
+        nothing
+    end
+end
+
+isdebugging() = false
+enable_debug() = (@eval isdebugging() = true; nothing)
+disable_debug() = (@eval isdebugging() = false; nothing)
diff --git a/src/quicksort.jl b/src/quicksort.jl
index 6e54c9fb..88b361cd 100644
--- a/src/quicksort.jl
+++ b/src/quicksort.jl
@@ -62,6 +62,7 @@ function _quicksort!(
             xs[end],
         ),
     )
+    chunksize = alg.basesize
 
     # TODO: Calculate extrema during the first pass if it's possible
     # to use counting sort.
@@ -69,33 +70,56 @@ function _quicksort!(
     # first pass.
 
     # Compute sizes of each partition for each chunks.
-    chunks = zip(_partition(xs, alg.basesize), _partition(cs, alg.basesize))
-    results = maptasks(partition_sizes!(pivot, order), chunks)
-    nbelows::Vector{Int} = map(first, results)
-    nequals::Vector{Int} = map(last, results)
-    naboves::Vector{Int} =
-        [length(c) - (b + e) for (b, e, (c, _)) in zip(nbelows, nequals, chunks)]
-    @check length(chunks) == length(nbelows) == length(nequals) == length(naboves)
-    @check all(>=(0), naboves)
-    singleton_chunkid = map(nbelows, nequals, naboves) do nb, ne, na
-        if (nb > 0) + (ne > 0) + (na > 0) == 1
-            return 1 * (nb > 0) + 2 * (ne > 0) + 3 * (na > 0)
-        else
-            return 0
-        end
+    xs_chunk_list = _partition(xs, chunksize)
+    cs_chunk_list = _partition(cs, chunksize)
+    nchunks = cld(length(xs), chunksize)
+    nbelows = Vector{Int}(undef, nchunks)
+    nequals = Vector{Int}(undef, nchunks)
+    naboves = Vector{Int}(undef, nchunks)
+    @DBG begin
+        VERSION >= v"1.4" &&
+            @check length(xs_chunk_list) == length(cs_chunk_list) == nchunks
+        fill!(nbelows, -1)
+        fill!(nequals, -1)
+        fill!(naboves, -1)
+    end
+    @sync for (nb, ne, na, xs_chunk, cs_chunk) in zip(
+        referenceable(nbelows),
+        referenceable(nequals),
+        referenceable(naboves),
+        xs_chunk_list,
+        cs_chunk_list,
+    )
+        @spawn partition_sizes!(nb, ne, na, xs_chunk, cs_chunk, pivot, order)
+    end
+    @DBG begin
+        @check all(>=(0), nbelows)
+        @check all(>=(0), nequals)
+        @check all(>=(0), naboves)
     end
 
-    below_offsets = copy(nbelows)
-    equal_offsets = copy(nequals)
-    above_offsets = copy(naboves)
+    below_offsets = nbelows
+    equal_offsets = nequals
+    above_offsets = naboves
     acc = exclusive_cumsum!(below_offsets)
     acc = exclusive_cumsum!(equal_offsets, acc)
     acc = exclusive_cumsum!(above_offsets, acc)
     @check acc == length(xs)
 
+    @inline function singleton_chunkid(i)
+        nb = @inbounds get(below_offsets, i + 1, equal_offsets[1]) - below_offsets[i]
+        ne = @inbounds get(equal_offsets, i + 1, above_offsets[1]) - equal_offsets[i]
+        na = @inbounds get(above_offsets, i + 1, length(ys)) - above_offsets[i]
+        if (nb > 0) + (ne > 0) + (na > 0) == 1
+            return 1 * (nb > 0) + 2 * (ne > 0) + 3 * (na > 0)
+        else
+            return 0
+        end
+    end
+
     @sync begin
-        for (i, (xs_chunk, cs_chunk)) in enumerate(chunks)
-            singleton_chunkid[i] > 0 && continue
+        for (i, (xs_chunk, cs_chunk)) in enumerate(zip(xs_chunk_list, cs_chunk_list))
+            singleton_chunkid(i) > 0 && continue
             @spawn unsafe_quicksort_scatter!(
                 ys,
                 xs_chunk,
@@ -105,13 +129,14 @@ function _quicksort!(
                 above_offsets[i],
             )
         end
-        for (i, (xs_chunk, _)) in enumerate(chunks)
-            singleton_chunkid[i] > 0 || continue
+        for (i, xs_chunk) in enumerate(xs_chunk_list)
+            sid = singleton_chunkid(i)
+            sid > 0 || continue
             idx = (
                 below_offsets[i]+1:get(below_offsets, i + 1, equal_offsets[1]),
                 equal_offsets[i]+1:get(equal_offsets, i + 1, above_offsets[1]),
                 above_offsets[i]+1:get(above_offsets, i + 1, length(ys)),
-            )[singleton_chunkid[i]]
+            )[sid]
             # There is only one partition. Short-circuit scattering.
             ys_chunk = view(ys, idx)
             copyto!(ys_chunk, xs_chunk)
@@ -226,7 +251,13 @@ function _quicksort_serial!(
     return ys_is_result ? ys : xs
 end
 
-partition_sizes!(pivot, order) = ((xs, cs),) -> partition_sizes!(xs, cs, pivot, order)
+function partition_sizes!(nbelows, nequals, naboves, xs, cs, pivot, order)
+    (nb, ne) = partition_sizes!(xs, cs, pivot, order)
+    nbelows[] = nb
+    nequals[] = ne
+    naboves[] = length(xs) - (nb + ne)
+    return
+end
 
 function partition_sizes!(xs, cs, pivot, order)
     nbelows = 0