Re: [rfc][patch] SLQB slab allocator

Nick Piggin
2008-12-12T05:50:51Z

On Fri, Dec 12, 2008 at 06:38:26AM +0100, Eric Dumazet wrote:
> Nick Piggin a écrit :
> > I'm going to continue working on this as I get time, and I plan to soon ask
> > to have it merged. It would be great if people could comment or test it.
> >
>
> It seems really good, but will need some hours to review :)
>
> Minor nit : You spelled Qeued instead of Queued in init/Kconfig
>
> +config SLQB
> + bool "SLQB (Qeued allocator)"

OK, thanks.

> One of the problem I see with SLAB & SLUB is the irq masking stuff.
> Some (many ???) kmem_cache are only used in process context, I see no point of
> disabling irqs for them.

That's a very good point actually, and something I want to look at...

I'm thinking it will make most sense to provide a
kmem_cache_alloc/free_irqsafe for callers who either don't do any
interrupt context allocations, or already have irqs off (another
slab flag will just add another branch in the fastpaths).

And then also a kmalloc/kfree_irqsoff for code which already has
irqs off.

That's something which benefit all slab allocators roughly equally,
so at the moment I'm concentrating on the core code. But it's a very
good idea.

> I tested your patch on my 8 ways HP BL460c G1, on top
> on my last patch serie. (linux-2.6, not net-next-2.6)
>
> # time ./socketallocbench
>
> real 0m1.300s
> user 0m0.078s
> sys 0m1.207s
> # time ./socketallocbench -n 8
>
> real 0m1.686s
> user 0m0.614s
> sys 0m12.737s
>
> So no bad effect (same than SLUB).

Cool, thanks.

> For the record, SLAB is really really bad for this workload
>
> PU: Core 2, speed 3000.1 MHz (estimated)
> Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit mask of 0x00 (Unhalted core cycles) count 100
> 000
> samples cum. samples % cum. % symbol name
> 136537 136537 10.8300 10.8300 kmem_cache_alloc
> 129380 265917 10.2623 21.0924 tcp_close
> 79696 345613 6.3214 27.4138 tcp_v4_init_sock
> 73873 419486 5.8596 33.2733 tcp_v4_destroy_sock
> 63436 482922 5.0317 38.3050 sysenter_past_esp
> 62140 545062 4.9289 43.2339 inet_csk_destroy_sock
> 56565 601627 4.4867 47.7206 kmem_cache_free
> 40430 642057 3.2069 50.9275 __percpu_counter_add
> 35742 677799 2.8350 53.7626 init_timer
> 35611 713410 2.8246 56.5872 copy_from_user
> 21616 735026 1.7146 58.3018 d_alloc
> 20821 755847 1.6515 59.9533 alloc_inode
> 19645 775492 1.5582 61.5115 alloc_fd
> 18935 794427 1.5019 63.0134 __fput
> 18922 813349 1.5009 64.5143 inet_create
> 18919 832268 1.5006 66.0149 sys_close
> 16074 848342 1.2750 67.2899 release_sock
> 15337 863679 1.2165 68.5064 lock_sock_nested
> 15172 878851 1.2034 69.7099 sock_init_data
> 14196 893047 1.1260 70.8359 fd_install
> 13677 906724 1.0849 71.9207 drop_file_write_access
> 13195 919919 1.0466 72.9673 dput
> 12768 932687 1.0127 73.9801 inotify_d_instantiate
> 11404 944091 0.9046 74.8846 init_waitqueue_head
> 11228 955319 0.8906 75.7752 sysenter_do_call
> 11213 966532 0.8894 76.6647 local_bh_enable_ip
> 10948 977480 0.8684 77.5330 __sock_create
> 10912 988392 0.8655 78.3986 local_bh_enable
> 10665 999057 0.8459 79.2445 __new_inode
> 10579 1009636 0.8391 80.0836 inet_release
> 9665 1019301 0.7666 80.8503 iput_single
> 9545 1028846 0.7571 81.6074 fput
> 7950 1036796 0.6306 82.2379 sock_release
> 7236 1044032 0.5740 82.8119 local_bh_disable
>
>
> We can see most of the time is taken by the memset() to clear object,
> then irq masking stuff...

Yep, it's difficult to make the local alloc/free fastpath much more
optimal as-is.

Is SLAB still bad at the test with the slab-rcu patch in place?
SLAB has a pretty optimal fastpath as well, although if its queues
start overflowing, it can run into contention quite easily.

>
> c0281e10 <kmem_cache_alloc>: /* kmem_cache_alloc total: 140659 10.8277 */

I guess you're compiling with -Os? I find gcc can pack the fastpath
much better with -O2, and actually decrease the effective icache
footprint size even if the total text size increases...

> 2414 0.1858 :c0281e10: push %ebp
> 7 5.4e-04 :c0281e11: mov %esp,%ebp
> :c0281e13: push %edi
> 1454 0.1119 :c0281e14: push %esi
> 310 0.0239 :c0281e15: mov %eax,%esi
> :c0281e17: push %ebx
> 368 0.0283 :c0281e18: sub $0x10,%esp
> 949 0.0731 :c0281e1b: mov %edx,-0x18(%ebp)
> 383 0.0295 :c0281e1e: mov 0x4(%ebp),%eax
> 1189 0.0915 :c0281e21: mov %eax,-0x14(%ebp)
> 1240 0.0955 :c0281e24: jmp c0281e6e <kmem_cache_alloc+0x5e>
> :c0281e26: lea 0x0(%esi),%esi
> :c0281e29: lea 0x0(%edi,%eiz,1),%edi
> 1188 0.0915 :c0281e30: mov 0x10(%esi),%eax
> :c0281e33: mov (%edx,%eax,1),%eax
> 1483 0.1142 :c0281e36: decl (%ebx)
> 898 0.0691 :c0281e38: mov %eax,0x4(%ebx)
> 586 0.0451 :c0281e3b: mov %edx,-0x1c(%ebp)
> 1 7.7e-05 :c0281e3e: pushl -0x10(%ebp)
> 1226 0.0944 :c0281e41: popf
> 26385 2.0311 :c0281e42: testl $0x210d00,(%esi)
> 1188 0.0915 :c0281e48: je c0281ef8 <kmem_cache_alloc+0xe8>
> :c0281e4e: mov -0x1c(%ebp),%eax
> :c0281e51: test %eax,%eax
> :c0281e53: je c0281ef8 <kmem_cache_alloc+0xe8>
> :c0281e59: mov -0x14(%ebp),%ecx
> :c0281e5c: mov -0x1c(%ebp),%edx
> :c0281e5f: mov %esi,%eax
> :c0281e61: call c0280d60 <alloc_debug_processing>
> :c0281e66: test %eax,%eax
> :c0281e68: jne c0281ef8 <kmem_cache_alloc+0xe8>
> 1205 0.0928 :c0281e6e: pushf
> 4888 0.3763 :c0281e6f: popl -0x10(%ebp)
> 319 0.0246 :c0281e72: cli
> 5975 0.4599 :c0281e73: nop
> :c0281e74: lea 0x0(%esi,%eiz,1),%esi
> :c0281e78: mov %fs:0xc068d004,%eax
> 1166 0.0898 :c0281e7e: mov 0x38(%esi,%eax,4),%ebx
> 26 0.0020 :c0281e82: mov 0x4(%ebx),%edx
> 662 0.0510 :c0281e85: test %edx,%edx
> :c0281e87: jne c0281e30 <kmem_cache_alloc+0x20>
> :c0281e89: mov 0xc(%ebx),%eax
> :c0281e8c: test %eax,%eax
> :c0281e8e: jne c0281ec8 <kmem_cache_alloc+0xb8>
> :c0281e90: mov %ebx,%edx
> :c0281e92: mov %esi,%eax
> :c0281e94: call c0280010 <__cache_list_get_page>
> :c0281e99: mov %eax,%edx
> :c0281e9b: test %eax,%eax
> :c0281e9d: jne c0281f31 <kmem_cache_alloc+0x121>
> :c0281ea3: mov $0xffffffff,%ecx
> 1 7.7e-05 :c0281ea8: mov -0x18(%ebp),%edx
> :c0281eab: mov %esi,%eax
> :c0281ead: call c02815a0 <__slab_alloc_page>
> :c0281eb2: test %eax,%eax
> :c0281eb4: jne c0281e78 <kmem_cache_alloc+0x68>
> :c0281eb6: movl $0x0,-0x1c(%ebp)
> :c0281ebd: jmp c0281e3e <kmem_cache_alloc+0x2e>
> :c0281ec2: lea 0x0(%esi),%esi
> :c0281ec8: mov %esi,%eax
> :c0281eca: mov %ebx,%edx
> :c0281ecc: call c0280240 <claim_remote_free_list>
> :c0281ed1: mov 0x4(%esi),%eax
> :c0281ed4: shl $0x2,%eax
> :c0281ed7: cmp %eax,(%ebx)
> :c0281ed9: ja c0281f48 <kmem_cache_alloc+0x138>
> :c0281edb: mov 0x4(%ebx),%edx
> :c0281ede: mov %edx,-0x1c(%ebp)
> :c0281ee1: test %edx,%edx
> :c0281ee3: je c0281e90 <kmem_cache_alloc+0x80>
> :c0281ee5: mov 0x10(%esi),%eax
> :c0281ee8: mov (%edx,%eax,1),%eax
> :c0281eeb: decl (%ebx)
> :c0281eed: mov %eax,0x4(%ebx)
> :c0281ef0: jmp c0281e3e <kmem_cache_alloc+0x2e>
>
> :c0281ef5: lea 0x0(%esi),%esi
> 1261 0.0971 :c0281ef8: cmpw $0x0,-0x18(%ebp)
> 957 0.0737 :c0281efd: jns c0281f26 <kmem_cache_alloc+0x116>
> 627 0.0483 :c0281eff: mov -0x1c(%ebp),%eax
> :c0281f02: test %eax,%eax
> :c0281f04: je c0281f26 <kmem_cache_alloc+0x116>
> 82 0.0063 :c0281f06: mov 0xc(%esi),%esi
> 2 1.5e-04 :c0281f09: mov -0x1c(%ebp),%ebx
> 527 0.0406 :c0281f0c: mov %esi,%ecx
> :c0281f0e: mov %ebx,%edi
> 86 0.0066 :c0281f10: shr $0x2,%ecx
> 602 0.0463 :c0281f13: xor %eax,%eax
> 1 7.7e-05 :c0281f15: mov %esi,%edx
> 74845 5.7614 :c0281f17: rep stos %eax,%es:(%edi)
> 1170 0.0901 :c0281f19: test $0x2,%dl
> 2 1.5e-04 :c0281f1c: je c0281f20 <kmem_cache_alloc+0x110>
> :c0281f1e: stos %ax,%es:(%edi)
> 600 0.0462 :c0281f20: test $0x1,%dl
> :c0281f23: je c0281f26 <kmem_cache_alloc+0x116>
> :c0281f25: stos %al,%es:(%edi)
> 1171 0.0901 :c0281f26: mov -0x1c(%ebp),%eax
> 199 0.0153 :c0281f29: add $0x10,%esp
> :c0281f2c: pop %ebx
> 2 1.5e-04 :c0281f2d: pop %esi
> 1215 0.0935 :c0281f2e: pop %edi
> 548 0.0422 :c0281f2f: leave
> 1251 0.0963 :c0281f30: ret
> :c0281f31: mov 0x10(%edx),%ecx
> :c0281f34: mov %ecx,-0x1c(%ebp)
> :c0281f37: mov 0x10(%esi),%eax
> :c0281f3a: mov (%ecx,%eax,1),%eax
> :c0281f3d: mov %eax,0x10(%edx)
> :c0281f40: jmp c0281e3e <kmem_cache_alloc+0x2e>
> :c0281f45: lea 0x0(%esi),%esi
> :c0281f48: mov %ebx,%edx
> :c0281f4a: mov %esi,%eax
> :c0281f4c: call c02811d0 <flush_free_list>
> :c0281f51: jmp c0281edb <kmem_cache_alloc+0xcb>
> :c0281f53: lea 0x0(%esi),%esi