Re: [RFC]: Support for zero-copy TCP transmit of user space data

Vladislav Bolkhovitin
2008-12-19T19:15:41Z

Jens Axboe, on 12/19/2008 10:07 PM wrote:
> On Fri, Dec 19 2008, Vladislav Bolkhovitin wrote:
>> David M. Lloyd, on 12/18/2008 09:43 PM wrote:
>>> On 12/18/2008 12:35 PM, Vladislav Bolkhovitin wrote:
>>>> An iSCSI target driver iSCSI-SCST was a part of the patchset
>>>> (http://lkml.org/lkml/2008/12/10/293). For it a nice optimization to
>>>> have TCP zero-copy transmit of user space data was implemented. Patch,
>>>> implementing this optimization was also sent in the patchset, see
>>>> http://lkml.org/lkml/2008/12/10/296.
>>> I'm probably ignorant of about 90% of the context here, but isn't this the
>>> sort of problem that was supposed to have been solved by vmsplice(2)?
>> No, vmsplice can't help here. ISCSI-SCST is a kernel space driver. But,
>> even if it was a user space driver, vmsplice wouldn't change anything
>> much. It doesn't have a possibility for a user to know, when
>> transmission of the data finished. So, it is intended to be used as:
>> vmsplice() buffer -> munmap() the buffer -> mmap() new buffer ->
>> vmsplice() it. But on the mmap() stage kernel has to zero all the newly
>> mapped pages and zeroing memory isn't much faster, than copying it.
>> Hence, there would be no considerable performance increase.
>
> vmsplice() isn't the right choice, but splice() very well could be. You
> could easily use splice internally as well. The vmsplice() part sort-of
> applies in the sense that you want to fill pages into a pipe, which is
> essentially what vmsplice() does. You'd need some helper to do that.

Sorry, Jens, but splice() works only if there is a file handle on the
another side, so user space doesn't see data buffers. But SCST needs to
serve a wider usage cases, like reading data with decompression from a
virtual tape, where decompression is done in user space. For those only
complete zero-copy network send, which I implemented, can give the best
performance.

> And
> the ack-on-xmit-done bits is something that splice-to-socket needs
> anyway, so I think it'd be quite a suitable choice for this.

So, are you writing that splice() could also benefit from the zero-copy
transmit feature, like I implemented?