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attachment:slab.stp [[attachment:slab.stp]]

Kernel cache memory allocation

From Ross Mikosh, mikosh@us.ibm.com

Problem

When it runs out of memory due to a memory leak, the OOM killer clock starts ticking. When the linux kernel runs out of memory, it starts killing processes. Next time, you are prepared. You monitor /proc/slabinfo and note one of the slabs is increasing and increasing and ... increasing some more. But why?

Scripts

slab.stp

global slab = @1
global stats, stacks
probe kernel.function("kmem_cache_alloc") {
        if (kernel_string($cachep->name) == slab) {
                stats[execname()] <<< 1
                stacks[execname(),kernel_string($cachep->name),backtrace()] <<< 1
        }
}
# Exit after 10 seconds
probe timer.ms(10000) { exit () }
probe end {
        printf("Number of %s slab allocations by process\n", slab)
        foreach ([exec] in stats) {
                printf("%s:\t%d\n",exec,@count(stats[exec]))
        }
        printf("\nBacktrace of processes when allocating\n")
        foreach ([proc,cache,bt] in stacks) {
                printf("Exec: %s Name: %s  Count: %d\n",proc,cache,@count(stacks[proc,cache,bt]))
                print_stack(bt)
                printf("\n-------------------------------------------------------\n\n")
        }
}

Output

# stap slab.stp size-32
Number of size-32 slab allocations by process
crond:  2
ps:     2
upsd:   1
snmpd:  3
httpd:  1
xfce4-panel:    1
xfce4-cpugraph-:        11
topfew: 4
Backtrace of processes when allocating
Exec: crond Name: size-32  Count: 2
 0xc045d717 : kmem_cache_alloc+0x2/0x4f []
 0xf890da44 : ext3_readdir+0x9c/0x5f4 [ext3]
 0xc04707d4 : vfs_readdir+0x66/0x90 []
 0xc0470a2b : sys_getdents+0x5f/0x9c []
 0xc0402d9b : syscall_call+0x7/0xb []
-------------------------------------------------------
[...]
Exec: httpd Name: size-32  Count: 1
 0xc045d717 : kmem_cache_alloc+0x2/0x4f []
 0xc045da31 : cache_alloc_refill+0x2cd/0x409 []
 0xc045dbd9 : __kmalloc+0x6c/0x76 []
 0xc05a19bb : pskb_expand_head+0x43/0x110 []
 0xc05a57a0 : skb_checksum_help+0x46/0xbc []
 0xf8a3d339 : ip_nat_fn+0x45/0x192 [iptable_nat]
 0xf8a3d6a6 : ip_nat_local_fn+0x3e/0xb0 [iptable_nat]
 0xc05baab8 : nf_iterate+0x38/0x6a []
 0xc05babf0 : nf_hook_slow+0x4d/0xb5 []
 0xc05c3ff1 : ip_queue_xmit+0x389/0x3ce []
 0xc05d1783 : tcp_transmit_skb+0x5f8/0x626 []
 0xc05d308f : __tcp_push_pending_frames+0x6bc/0x783 []
 0xc05d3e7e : tcp_send_fin+0x13b/0x141 []
 0xc05dfeac : inet_shutdown+0x80/0xbb []
 0x0000000c
[...]

Lessons

Not too hard.


None: WSKmemCacheAlloc (last edited 2008-01-10 19:47:26 by localhost)