[lustre-devel] [PATCH 24/25] staging: lustre: libcfs: change CPT estimate algorithm

[James Simmons <jsimmons@infradead.org>]

From: Dmitry Eremin <dmitry.eremin@intel.com>

The main idea to have more CPU partitions is based on KNL experience.
When a thread submit IO for network communication one of threads from
current CPT is used for network stack. Whith high parallelization many
threads become involved in network submission but having less CPU
partitions they will wait until single thread process them from network
queue. So, the bottleneck just moves into network layer in case of
small amount of CPU partitions. My experiments showed that the best
performance was when for each IO thread we have one network thread.
This condition can be provided having 2 real HW cores (without hyper
threads) per CPT. This is exactly what implemented in this patch.

Change CPT estimate algorithm from 2 * (N - 1)^2 < NCPUS <= 2 * N^2
to 2 HW cores per CPT. This is critical for machines with number of
cores different from 2^N.

Current algorithm splits CPTs in KNL:
LNet: HW CPU cores: 272, npartitions: 16
cpu_partition_table=
    0       : 0-4,68-71,136-139,204-207
    1       : 5-9,73-76,141-144,209-212
    2       : 10-14,78-81,146-149,214-217
    3       : 15-17,72,77,83-85,140,145,151-153,208,219-221
    4       : 18-21,82,86-88,150,154-156,213,218,222-224
    5       : 22-26,90-93,158-161,226-229
    6       : 27-31,95-98,163-166,231-234
    7       : 32-35,89,100-103,168-171,236-239
    8       : 36-38,94,99,104-105,157,162,167,172-173,225,230,235,240-241
    9       : 39-43,107-110,175-178,243-246
    10      : 44-48,112-115,180-183,248-251
    11      : 49-51,106,111,117-119,174,179,185-187,242,253-255
    12      : 52-55,116,120-122,184,188-190,247,252,256-258
    13      : 56-60,124-127,192-195,260-263
    14      : 61-65,129-132,197-200,265-268
    15      : 66-67,123,128,133-135,191,196,201-203,259,264,269-271

New algorithm will split CPTs in KNL:
LNet: HW CPU cores: 272, npartitions: 34
cpu_partition_table=
    0       : 0-1,68-69,136-137,204-205
    1       : 2-3,70-71,138-139,206-207
    2       : 4-5,72-73,140-141,208-209
    3       : 6-7,74-75,142-143,210-211
    4       : 8-9,76-77,144-145,212-213
    5       : 10-11,78-79,146-147,214-215
    6       : 12-13,80-81,148-149,216-217
    7       : 14-15,82-83,150-151,218-219
    8       : 16-17,84-85,152-153,220-221
    9       : 18-19,86-87,154-155,222-223
    10      : 20-21,88-89,156-157,224-225
    11      : 22-23,90-91,158-159,226-227
    12      : 24-25,92-93,160-161,228-229
    13      : 26-27,94-95,162-163,230-231
    14      : 28-29,96-97,164-165,232-233
    15      : 30-31,98-99,166-167,234-235
    16      : 32-33,100-101,168-169,236-237
    17      : 34-35,102-103,170-171,238-239
    18      : 36-37,104-105,172-173,240-241
    19      : 38-39,106-107,174-175,242-243
    20      : 40-41,108-109,176-177,244-245
    21      : 42-43,110-111,178-179,246-247
    22      : 44-45,112-113,180-181,248-249
    23      : 46-47,114-115,182-183,250-251
    24      : 48-49,116-117,184-185,252-253
    25      : 50-51,118-119,186-187,254-255
    26      : 52-53,120-121,188-189,256-257
    27      : 54-55,122-123,190-191,258-259
    28      : 56-57,124-125,192-193,260-261
    29      : 58-59,126-127,194-195,262-263
    30      : 60-61,128-129,196-197,264-265
    31      : 62-63,130-131,198-199,266-267
    32      : 64-65,132-133,200-201,268-269
    33      : 66-67,134-135,202-203,270-271

'N' pattern in KNL works is not always good.
in flat mode it will be one CPT with all CPUs inside.

in SNC-4 mode:
cpu_partition_table=
    0       : 0-17,68-85,136-153,204-221
    1       : 18-35,86-103,154-171,222-239
    2       : 36-51,104-119,172-187,240-255
    3       : 52-67,120-135,188-203,256-271

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8703
Reviewed-on: https://review.whamcloud.com/24304
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
---
 .../staging/lustre/lnet/libcfs/linux/linux-cpu.c   | 30 ++++------------------
 1 file changed, 5 insertions(+), 25 deletions(-)

diff --git a/drivers/staging/lustre/lnet/libcfs/linux/linux-cpu.c b/drivers/staging/lustre/lnet/libcfs/linux/linux-cpu.c
index 915cfca..ae5fd16 100644
--- a/drivers/staging/lustre/lnet/libcfs/linux/linux-cpu.c
+++ b/drivers/staging/lustre/lnet/libcfs/linux/linux-cpu.c
@@ -768,34 +768,14 @@ static int cfs_cpt_choose_ncpus(struct cfs_cpt_table *cptab, int cpt,
 
 static int cfs_cpt_num_estimate(void)
 {
-	int nnode = num_online_nodes();
+	int nthr = cpumask_weight(topology_sibling_cpumask(smp_processor_id()));
 	int ncpu = num_online_cpus();
-	int ncpt;
+	int ncpt = 1;
 
-	if (ncpu <= CPT_WEIGHT_MIN) {
-		ncpt = 1;
-		goto out;
-	}
-
-	/* generate reasonable number of CPU partitions based on total number
-	 * of CPUs, Preferred N should be power2 and match this condition:
-	 * 2 * (N - 1)^2 < NCPUS <= 2 * N^2
-	 */
-	for (ncpt = 2; ncpu > 2 * ncpt * ncpt; ncpt <<= 1)
-		;
-
-	if (ncpt <= nnode) { /* fat numa system */
-		while (nnode > ncpt)
-			nnode >>= 1;
+	if (ncpu > CPT_WEIGHT_MIN)
+		for (ncpt = 2; ncpu > 2 * nthr * ncpt; ncpt++)
+			; /* nothing */
 
-	} else { /* ncpt > nnode */
-		while ((nnode << 1) <= ncpt)
-			nnode <<= 1;
-	}
-
-	ncpt = nnode;
-
-out:
 #if (BITS_PER_LONG == 32)
 	/* config many CPU partitions on 32-bit system could consume
 	 * too much memory
-- 
1.8.3.1