[RFC PATCH v2 0/6] Reduce cache miss for snmp_fold_field

Jia He
2016-09-06T02:30:23Z

In a PowerPc server with large cpu number(160), besides commit
a3a773726c9f ("net: Optimize snmp stat aggregation by walking all
the percpu data at once"), I watched several other snmp_fold_field
callsites which will cause high cache miss rate.

My simple test case, which read from the procfs items endlessly:
/***********************************************************/
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#define LINELEN 2560
int main(int argc, char **argv)
{
int i;
int fd = -1 ;
int rdsize = 0;
char buf[LINELEN+1];

buf[LINELEN] = 0;
memset(buf,0,LINELEN);

if(1 >= argc) {
printf("file name empty\n");
return -1;
}

fd = open(argv[1], O_RDWR, 0644);
if(0 > fd){
printf("open error\n");
return -2;
}

for(i=0;i<0xffffffff;i++) {
while(0 < (rdsize = read(fd,buf,LINELEN))){
//nothing here
}

lseek(fd, 0, SEEK_SET);
}

close(fd);
return 0;
}
/**********************************************************/

compile and run:
gcc test.c -o test

perf stat -d -e cache-misses ./test /proc/net/snmp
perf stat -d -e cache-misses ./test /proc/net/snmp6
perf stat -d -e cache-misses ./test /proc/net/netstat
perf stat -d -e cache-misses ./test /proc/net/sctp/snmp
perf stat -d -e cache-misses ./test /proc/net/xfrm_stat

before the patch set:
====================
Performance counter stats for 'system wide':

355911097 cache-misses [40.08%]
2356829300 L1-dcache-loads [60.04%]
355642645 L1-dcache-load-misses # 15.09% of all L1-dcache hits [60.02%]
346544541 LLC-loads [59.97%]
389763 LLC-load-misses # 0.11% of all LL-cache hits [40.02%]

6.245162638 seconds time elapsed

After the patch set:
===================
Performance counter stats for 'system wide':

194992476 cache-misses [40.03%]
6718051877 L1-dcache-loads [60.07%]
194871921 L1-dcache-load-misses # 2.90% of all L1-dcache hits [60.11%]
187632232 LLC-loads [60.04%]
464466 LLC-load-misses # 0.25% of all LL-cache hits [39.89%]

6.868422769 seconds time elapsed
The cache-miss rate can be reduced from 15% to 2.9%

v2:
- 1/6 fix bug in udplite statistics.
- 1/6 snmp_seq_show is split into 2 parts

Jia He (6):
proc: Reduce cache miss in {snmp,netstat}_seq_show
proc: Reduce cache miss in snmp6_seq_show
proc: Reduce cache miss in sctp_snmp_seq_show
proc: Reduce cache miss in xfrm_statistics_seq_show
ipv6: Remove useless parameter in __snmp6_fill_statsdev
net: Suppress the "Comparison to NULL could be written" warning

net/ipv4/proc.c | 144 ++++++++++++++++++++++++++++++++++-----------------
net/ipv6/addrconf.c | 12 ++---
net/ipv6/proc.c | 47 +++++++++++++----
net/sctp/proc.c | 15 ++++--
net/xfrm/xfrm_proc.c | 15 ++++--
5 files changed, 162 insertions(+), 71 deletions(-)

--
1.8.3.1