Address poor performance of Parquet string decoding (#15304)

See #15297. The Parquet string decoder can become a bottleneck in the presence of strings of widely varying sizes. This PR is an attempt to address this, at least as a stop gap solution. A more complete solution may be to rework the string decoder to work in a block-wide fashion, such as the new micro-kernels added in #15159.

Authors:
  - Ed Seidl (https://github.com/etseidl)
  - Nghia Truong (https://github.com/ttnghia)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Vukasin Milovanovic (https://github.com/vuule)

URL: #15304

cpp/src/io/parquet/page_string_decode.cu

@@ -1045,12 +1045,6 @@ CUDF_KERNEL void __launch_bounds__(decode_block_size)
       //
       if (!has_repetition) { dst_pos -= s->first_row; }
 
-      // need to do this before we branch on src_pos/dst_pos so we don't deadlock
-      // choose a character parallel string copy when the average string is longer than a warp
-      using cudf::detail::warp_size;
-      auto const use_char_ll =
-        s->page.num_valids > 0 && (s->page.str_bytes / s->page.num_valids) >= warp_size;
-
       if (me < warp_size) {
         for (int i = 0; i < decode_block_size - out_thread0; i += warp_size) {
           dst_pos = sb->nz_idx[rolling_index<rolling_buf_size>(src_pos + i)];
@@ -1061,10 +1055,13 @@ CUDF_KERNEL void __launch_bounds__(decode_block_size)
                               : cuda::std::pair<char const*, size_t>{nullptr, 0};
 
           __shared__ cub::WarpScan<size_type>::TempStorage temp_storage;
-          size_type offset;
-          cub::WarpScan<size_type>(temp_storage).ExclusiveSum(len, offset);
+          size_type offset, warp_total;
+          cub::WarpScan<size_type>(temp_storage).ExclusiveSum(len, offset, warp_total);
           offset += last_offset;
 
+          // choose a character parallel string copy when the average string is longer than a warp
+          auto const use_char_ll = warp_total / warp_size >= warp_size;
+
           if (use_char_ll) {
             __shared__ __align__(8) uint8_t const* pointers[warp_size];
             __shared__ __align__(4) size_type offsets[warp_size];