Re: ZFS txg implementation flaw

From: Xin Li <delphij_at_delphij.net>
Date: Mon, 28 Oct 2013 14:56:17 -0700
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On 10/28/13 14:45, Slawa Olhovchenkov wrote:
> On Mon, Oct 28, 2013 at 02:38:30PM -0700, Xin Li wrote:
> 
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>> On 10/28/13 14:32, Slawa Olhovchenkov wrote:
>>> On Mon, Oct 28, 2013 at 02:22:16PM -0700, Jordan Hubbard
>>> wrote:
>>> 
>>>> 
>>>> On Oct 28, 2013, at 2:28 AM, Slawa Olhovchenkov
>>>> <slw_at_zxy.spb.ru> wrote:
>>>> 
>>>>> As I see ZFS cretate seperate thread for earch txg writing.
>>>>>  Also for writing to L2ARC. As result -- up to several
>>>>> thousands threads created and destoyed per second. And
>>>>> hundreds thousands page allocations, zeroing, maping
>>>>> unmaping and freeing per seconds. Very high overhead.
>>>> 
>>>> How are you measuring the number of threads being created / 
>>>> destroyed?   This claim seems erroneous given how the ZFS
>>>> thread pool mechanism actually works (and yes, there are
>>>> thread pools already).
>>>> 
>>>> It would be helpful to both see your measurement methodology
>>>> and the workload you are using in your tests.
>>> 
>>> Semi-indirect. dtrace -n 'fbt:kernel:vm_object_terminate:entry
>>> { _at_traces[stack()] = count(); }'
>>> 
>>> After some (2-3) seconds
>>> 
>>> kernel`vnode_destroy_vobject+0xb9
>>> zfs.ko`zfs_freebsd_reclaim+0x2e kernel`VOP_RECLAIM_APV+0x78
>>> kernel`vgonel+0x134 kernel`vnlru_free+0x362
>>> kernel`vnlru_proc+0x61e kernel`fork_exit+0x11f
>>> kernel`0xffffffff80cdbfde 2490
> 
> 0xffffffff80cdbfd0 <fork_trampoline>:   mov    %r12,%rdi 
> 0xffffffff80cdbfd3 <fork_trampoline+3>: mov    %rbx,%rsi 
> 0xffffffff80cdbfd6 <fork_trampoline+6>: mov    %rsp,%rdx 
> 0xffffffff80cdbfd9 <fork_trampoline+9>: callq  0xffffffff808db560
> <fork_exit> 0xffffffff80cdbfde <fork_trampoline+14>:        jmpq
> 0xffffffff80cdca80 <doreti> 0xffffffff80cdbfe3
> <fork_trampoline+19>:        nopw 0x0(%rax,%rax,1) 
> 0xffffffff80cdbfe9 <fork_trampoline+25>:        nopl   0x0(%rax)
> 
> 
>>> I don't have user process created threads nor do fork/exit.
>> 
>> This has nothing to do with fork/exit but does suggest that you
>> are running of vnodes.  What does sysctl -a | grep vnode say?
> 
> kern.maxvnodes: 1095872 kern.minvnodes: 273968 
> vm.stats.vm.v_vnodepgsout: 0 vm.stats.vm.v_vnodepgsin: 62399 
> vm.stats.vm.v_vnodeout: 0 vm.stats.vm.v_vnodein: 10680 
> vfs.freevnodes: 275107 vfs.wantfreevnodes: 273968 vfs.numvnodes:
> 316321 debug.sizeof.vnode: 504

Try setting vfs.wantfreevnodes to 547936 (double it).

Cheers,
- -- 
Xin LI <delphij_at_delphij.net>    https://www.delphij.net/
FreeBSD - The Power to Serve!           Live free or die
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Received on Mon Oct 28 2013 - 20:56:18 UTC

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