Messages in this thread | | | Date | Fri, 19 Dec 2008 22:17:53 +0300 | From | Vladislav Bolkhovitin <> | Subject | Re: [RFC]: Support for zero-copy TCP transmit of user space data |
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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?
Thanks, Vlad
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