Messages in this thread | | | Subject | Re: [PATCH net-next 0/3] vhost: accelerate metadata access through vmap() | From | Jason Wang <> | Date | Wed, 26 Dec 2018 11:59:45 +0800 |
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On 2018/12/25 下午8:52, Michael S. Tsirkin wrote: > On Tue, Dec 25, 2018 at 06:09:19PM +0800, Jason Wang wrote: >> On 2018/12/25 上午2:12, Michael S. Tsirkin wrote: >>> On Mon, Dec 24, 2018 at 04:32:39PM +0800, Jason Wang wrote: >>>> On 2018/12/14 下午8:33, Michael S. Tsirkin wrote: >>>>> On Fri, Dec 14, 2018 at 11:42:18AM +0800, Jason Wang wrote: >>>>>> On 2018/12/13 下午11:27, Michael S. Tsirkin wrote: >>>>>>> On Thu, Dec 13, 2018 at 06:10:19PM +0800, Jason Wang wrote: >>>>>>>> Hi: >>>>>>>> >>>>>>>> This series tries to access virtqueue metadata through kernel virtual >>>>>>>> address instead of copy_user() friends since they had too much >>>>>>>> overheads like checks, spec barriers or even hardware feature >>>>>>>> toggling. >>>>>>> Userspace accesses through remapping tricks and next time there's a need >>>>>>> for a new barrier we are left to figure it out by ourselves. >>>>>> I don't get here, do you mean spec barriers? >>>>> I mean the next barrier people decide to put into userspace >>>>> memory accesses. >>>>> >>>>>> It's completely unnecessary for >>>>>> vhost which is kernel thread. >>>>> It's defence in depth. Take a look at the commit that added them. >>>>> And yes quite possibly in most cases we actually have a spec >>>>> barrier in the validation phase. If we do let's use the >>>>> unsafe variants so they can be found. >>>> unsafe variants can only work if you can batch userspace access. This is not >>>> necessarily the case for light load. >>> Do we care a lot about the light load? How would you benchmark it? >>> >> If we can reduce the latency that's will be more than what we expect. >> >> 1 byte TCP_RR shows 1.5%-2% improvement. > It's nice but not great. E.g. adaptive polling would be > a better approach to work on latency imho.
Actually this is another advantages of vmap():
For split ring, we will poll avail idx
For packed ring, we will poll wrap counter
Either of which can not be batched.
> >>>>>> And even if you're right, vhost is not the >>>>>> only place, there's lots of vmap() based accessing in kernel. >>>>> For sure. But if one can get by without get user pages, one >>>>> really should. Witness recently uncovered mess with file >>>>> backed storage. >>>> We only pin metadata pages, I don't believe they will be used by any DMA. >>> It doesn't matter really, if you dirty pages behind the MM back >>> the problem is there. >> >> Ok, but the usual case is anonymous pages, do we use file backed pages for >> user of vhost? > Some people use file backed pages for vms. > Nothing prevents them from using vhost as well.
Ok.
> >> And even if we use sometime, according to the pointer it's >> not something that can fix, RFC has been posted to solve this issue. >> >> Thanks > Except it's not broken if we don't to gup + write. > So yea, wait for rfc to be merged. >
Yes.
Thanks
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