Remove srvIOThreadPool and its throw-away datagen + compression (facebookresearch#728)

Summary:
Pull Request resolved: facebookresearch#728

The srvIOThread pool was a synthetic CPU placeholder for outbound RPC work back when LeafNodeRank had no real outbound RPCs: each request would fan out N tasks onto srvIOThread, each of which generated a fake response, serialized it, ran ZSTD over half the chain, and discarded the result. With Phase 5 in place, the SREventBase pool now carries real outbound RPC fanout to mock_services (issueOutboundFanout), so the placeholder is redundant and was actively skewing the prod-vs-bench category breakdown by overweighting Compression (8-13% bench vs 3-5% prod).

This commit:
* Drops the srvIOThread pool construction, warmup, and the --srv_io_threads CLI flag.
* Removes the per-request throw-away generateResponse + serializePayload + compressThrift fanout from AsyncPageRankRequestHandler, DLRMRequestHandler (async chain), and PageRankRequestHandler (sync). The single response we actually send back to the driver is still generated and serialized — just once instead of srv_io_threads times.
* Removes the unused dispatcher-thread `compressed = compressPayload(...)` / `decompressPayload(compressed)` pair from the sync PageRankRequestHandler that was paired with the same throw-away pattern.
* Removes srvIOThreadPool from ThreadData and from both ThreadStartup overloads.
* Removes -s / --srv_io_threads handling from packages/feedsim/run.sh and the static --srv_io_threads=36 from start_leaf_node_rank.sh.

The SREventBase pool's comment is updated from "idle in Phase 4" to its actual role (carries outbound fanout), and the startup banner now lists 4 pools instead of 5.

Reviewed By: YifanYuan3

Differential Revision: D105119222

Author: excelle08

packages/feedsim/run.sh

@@ -50,13 +50,11 @@ EVENTBASE_THREADS_DEFAULT=4  # 4 should suffice. Tune up if threads are saturate
 SRV_THREADS_DEFAULT=8        # 8 should also suffice for most purposes
 if [[ "$IS_SMT_ON" = 1 ]]; then
   RANKING_THREADS_DEFAULT="$(( $(nproc) * 7/20))"  # 7/20 is 0.35 cpu factor
-  SRV_IO_THREADS_DEFAULT="$(echo "${BC_MIN_FN}; min($(nproc) * 7/20, 55)" | bc)" # 0.35 cpu factor, max 55
   DRIVER_THREADS="$(echo "scale=2; $(nproc) / 5.0 + 0.5 " | bc )"  # Driver threads, rounds nearest.
   DRIVER_THREADS="${DRIVER_THREADS%.*}"  # Truncate decimal fraction.
   DRIVER_THREADS="$(echo "${BC_MAX_FN}; max(${DRIVER_THREADS:-0}, 4)" | bc )" # At least 4 threads.
 else
   RANKING_THREADS_DEFAULT="$(( $(nproc) * 15/20))"  # 15/20 is 0.75 cpu factor
-  SRV_IO_THREADS_DEFAULT="$(echo "${BC_MIN_FN}; min($(nproc) * 11/20, 55)" | bc)" # 0.55 cpu factor, max 55
   DRIVER_THREADS="$(echo "scale=2; $(nproc) / 4.0 + 0.5 " | bc )"  # Driver threads, rounds nearest.
   DRIVER_THREADS="${DRIVER_THREADS%.*}"  # Truncate decimal fraction.
   DRIVER_THREADS="$(echo "${BC_MAX_FN}; max(${DRIVER_THREADS:-0}, 4)" | bc )" # At least 4 threads.
@@ -69,7 +67,6 @@ Usage: ${0##*/} [OPTION]...
     -h Display this help and exit
     -t Number of threads to use for thrift serving. Large dataset kept per thread. Default: $THRIFT_THREADS_DEFAULT
     -c Number of threads to use for fanout ranking work. Heavy CPU work. Default: $RANKING_THREADS_DEFAULT
-    -s Number of threads to use for task-based serialization cpu work. Default: $SRV_IO_THREADS_DEFAULT
     -l Number of threads to use for load generation in the drivers. Default: $DRIVER_THREADS
     -a When searching for the optimal QPS, automatically adjust the number of client driver threads by
        min(requested_qps / 4, $(nproc) / 5) in each iteration (experimental feature).
@@ -146,9 +143,6 @@ main() {
     local ranking_cpu_threads
     ranking_cpu_threads="$RANKING_THREADS_DEFAULT"
 
-    local srv_io_threads
-    srv_io_threads="$SRV_IO_THREADS_DEFAULT"
-
     local driver_threads
     driver_threads="$DRIVER_THREADS"
 
@@ -303,10 +297,6 @@ main() {
                 ranking_cpu_threads="$2"
                 shift
                 ;;
-            -s)
-                srv_io_threads="$2"
-                shift
-                ;;
             -l)
                 driver_threads="$2"
                 shift
@@ -724,7 +714,6 @@ main() {
         --timekeeper_threads=2 \
         --io_threads="$EVENTBASE_THREADS_DEFAULT" \
         --srv_threads="$SRV_THREADS_DEFAULT" \
-        --srv_io_threads="$srv_io_threads" \
         --num_objects=2000 \
         --graph_max_iters=1 \
         --noaffinity \

packages/feedsim/third_party/src/scripts/start_leaf_node_rank.sh

@@ -12,7 +12,6 @@
      --timekeeper_threads=2 \
      --io_threads=4 \
      --srv_threads=8 \
-     --srv_io_threads=36 \
      --num_objects=2000 \
      --graph_max_iters=1 \
      --noaffinity \