Re: [PATCH] aio: convert the ioctx list to radix tree

[Octavian Purdila]

On Fri, Mar 22, 2013 at 3:05 PM, Zach Brown <zab@xxxxxxxxxx> wrote:
> On Fri, Mar 22, 2013 at 08:33:19PM +0200, Octavian Purdila wrote:
>> When using a large number of threads performing AIO operations the
>> IOCTX list may get a significant number of entries which will cause
>> significant overhead. For example, when running this fio script:
>
> Indeed.  But you also need to consider the impact this change has on the
> typical case of only having one ctx in the mm.  Please include
> measurements of that case in the commit message.
>

Hi Zach,

Good point, I will add those numbers.

>> --- a/arch/s390/mm/pgtable.c
>> +++ b/arch/s390/mm/pgtable.c
>> @@ -831,7 +831,7 @@ int s390_enable_sie(void)
>>       task_lock(tsk);
>>       if (!tsk->mm || atomic_read(&tsk->mm->mm_users) > 1 ||
>>  #ifdef CONFIG_AIO
>> -         !hlist_empty(&tsk->mm->ioctx_list) ||
>> +         tsk->mm->ioctx_rtree.rnode ||
>>  #endif
>
> Boy, what a curious thing.  I wonder if this is still needed if we're no
> longer storing the mm in the ctx after retry support is removed.
>
>> +     err = radix_tree_insert(&mm->ioctx_rtree, ctx->user_id, ctx);
>
> Hmm.  Is there anything stopping an exceptionally jerky app from racing
> io_setup() and munmap() and having two contexts be mapped to the same
> address and get the same user_id?  I guess this would just return
> -EEXIST, then, not do anything terrible.  I guess that's OK?
>

Ah, interesting, didn't thought of that. But I guess it is OK to keep
the WARN_ONCE there, as the application is not supposed to do the
munmap.

>> +                                            idx, sizeof(ctx)/sizeof(void *));
>
> ARRAY_SIZE(ctx)
>
> And why bother tracking the starting idx?  If you're completely draining
> it simply always start from 0?
>

I will fix these in the next version, thanks for reviewing !
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