Re: [PATCH v10 01/17] iova: Export alloc_iova_fast() and free_iova_fast()

[Jason Wang <jasowang@redhat.com>]

在 2021/8/9 下午1:56, Yongji Xie 写道:
> On Thu, Aug 5, 2021 at 9:31 PM Jason Wang <jasowang@redhat.com> wrote:
>>
>> 在 2021/8/5 下午8:34, Yongji Xie 写道:
>>>> My main point, though, is that if you've already got something else
>>>> keeping track of the actual addresses, then the way you're using an
>>>> iova_domain appears to be something you could do with a trivial bitmap
>>>> allocator. That's why I don't buy the efficiency argument. The main
>>>> design points of the IOVA allocator are to manage large address spaces
>>>> while trying to maximise spatial locality to minimise the underlying
>>>> pagetable usage, and allocating with a flexible limit to support
>>>> multiple devices with different addressing capabilities in the same
>>>> address space. If none of those aspects are relevant to the use-case -
>>>> which AFAICS appears to be true here - then as a general-purpose
>>>> resource allocator it's rubbish and has an unreasonably massive memory
>>>> overhead and there are many, many better choices.
>>>>
>>> OK, I get your point. Actually we used the genpool allocator in the
>>> early version. Maybe we can fall back to using it.
>>
>> I think maybe you can share some perf numbers to see how much
>> alloc_iova_fast() can help.
>>
> I did some fio tests[1] with a ram-backend vduse block device[2].
>
> Following are some performance data:
>
>                              numjobs=1   numjobs=2    numjobs=4   numjobs=8
> iova_alloc_fast    145k iops      265k iops      514k iops      758k iops
>
> iova_alloc            137k iops     170k iops      128k iops      113k iops
>
> gen_pool_alloc   143k iops      270k iops      458k iops      521k iops
>
> The iova_alloc_fast() has the best performance since we always hit the
> per-cpu cache. Regardless of the per-cpu cache, the genpool allocator
> should be better than the iova allocator.


I think we see convincing numbers for using iova_alloc_fast() than the 
gen_poll_alloc() (45% improvement on job=8).

Thanks


>
> [1] fio jobfile:
>
> [global]
> rw=randread
> direct=1
> ioengine=libaio
> iodepth=16
> time_based=1
> runtime=60s
> group_reporting
> bs=4k
> filename=/dev/vda
> [job]
> numjobs=..
>
> [2]  $ qemu-storage-daemon \
>        --chardev socket,id=charmonitor,path=/tmp/qmp.sock,server,nowait \
>        --monitor chardev=charmonitor \
>        --blockdev
> driver=host_device,cache.direct=on,aio=native,filename=/dev/nullb0,node-name=disk0
> \
>        --export type=vduse-blk,id=test,node-name=disk0,writable=on,name=vduse-null,num-queues=16,queue-size=128
>
> The qemu-storage-daemon can be builded based on the repo:
> https://github.com/bytedance/qemu/tree/vduse-test.
>
> Thanks,
> Yongji
>