Single threaded v multithreaded Thunderbolt 3 + Migration

[RumorConsumer]

I recently had an issue with migrating between two very recent Macs:

Mac 1: 2017 3.5ghz 13" MBP w 16gb RAM 512gb SSD
Mac 2: 2018 2.6ghz 15" MBP w same stats

I have a Belkin USBC-USBC cable that I have used in conjunction w an external SSD with speeds over 500mb/sec so I know it works. I have migrated from a 2015 2.5ghz MBP to the 13" at breakneck (450mb/sec+) speeds so I know its possible, but for some reason using the Migration Assistant on BOTH SIDES (not Target Disk Mode, though that had been my preferred method a while ago) I get a very low speed transfer on the *exact same data* - between 90mb/sec - 140mb/sec.

I submitted this as a bug and though I am still in the conversation I got this interesting reply:

"Note that TDM speeds will not approximate that of of standard TBT due to the connection being single threaded through EFI. We would expect and have seen approximately 260-280MB/sec in our testing."

I had never thought that EFI would only allow a single threaded connection but that makes sense in some way. This is different than TB2, I gather. Anybody have light to shed on that whole deal or the nature of my problem? Looking around the web there does seem to be a smattering of reports of slow migrations. Has anybody done TB3-TB3 over USBC w blazing fast speeds between two TB3 Macs?

 

[1096bimu]

Just because you cable supports faster standards doesn't mean Migration assistant will use it. There might be technical reasons why it won't use it.
Full TB3 would be more like 3000mb/s I don't know where you got that 260mb/s from.

 

[green86]

I recently had an issue with migrating between two very recent Macs:

Mac 1: 2017 3.5ghz 13" MBP w 16gb RAM 512gb SSD
Mac 2: 2018 2.6ghz 15" MBP w same stats

I have a Belkin USBC-USBC cable that I have used in conjunction w an external SSD with speeds over 500mb/sec so I know it works. I have migrated from a 2015 2.5ghz MBP to the 13" at breakneck (450mb/sec+) speeds so I know its possible, but for some reason using the Migration Assistant on BOTH SIDES (not Target Disk Mode, though that had been my preferred method a while ago) I get a very low speed transfer on the *exact same data* - between 90mb/sec - 140mb/sec.

I submitted this as a bug and though I am still in the conversation I got this interesting reply:

"Note that TDM speeds will not approximate that of of standard TBT due to the connection being single threaded through EFI. We would expect and have seen approximately 260-280MB/sec in our testing."

I had never thought that EFI would only allow a single threaded connection but that makes sense in some way. This is different than TB2, I gather. Anybody have light to shed on that whole deal or the nature of my problem? Looking around the web there does seem to be a smattering of reports of slow migrations. Has anybody done TB3-TB3 over USBC w blazing fast speeds between two TB3 Macs?

I'm not really sure where to start. I'm also not sure what TDM means other then time division multiplexing, is that what your abbreviating?

You can't just use any USB C cable, and if you only were getting 500 MBs it may not support Thunderbolt 3. So you may have been transferring data using the USB bus. When you did the 2015, did you use a thunderbolt cable? If so, part of the reason could be attributed to the overhead used by usb-c (really, just usb 3). USB uses processor cycles, and the processors were busy doing more then just copying stuff over; migration assistant checks for potential problems, versions conflicts and the like. I would not say 140 is slow, just slower then before and really, even if you had all 512 GB full (which with formatting is not possible) your talking about a migration that took an hour (512,000 / 140 = 3657 seconds, / 60 = 60 minutes) quite inline with migrations I've done, and I've done plenty.

 

[maflynn]

I'm also not sure what TDM means other then time division multiplexing

Given the context of the OP's post, I'd say its Target Disk Mode.

 

[RumorConsumer]

Just because you cable supports faster standards doesn't mean Migration assistant will use it. There might be technical reasons why it won't use it.
Full TB3 would be more like 3000mb/s I don't know where you got that 260mb/s from.

To clarify, I did not write that. That is a reply from an Apple response engineer. They appear to be saying that in Target Disk Mode (TDM, as @maflynn correctly discerned) you only get a single threaded channel of Thunderbolt 3 which comes in at those speeds. I will need to test that in order to verify it, but it sounds somewhat suspect to me as well.

Also, I would be shocked if Migration Assistant had any say at the link speed negotiation level. It does correctly report Thunderbolt as the connection type. I didn't post this initially bc I didnt think it would come up but here is an image of the machines mid-transfer while at sub-standard speeds. You can clearly see the Thunderbolt logo under connection type.

@green86 Yes, while there is some variation in USB-C cables I am using the Apple Store sold Belkin USB-C to USB-C cable. This one: https://www.belkin.com/us/p/P-F2CU030/

It is certified to do it all. The cable was not the issue though I could try another one I suppose.
To your point about USB overhead I just don't see that man. That was true of iMac G3s but we are talking about top of the line Macs of the last 12 months. USB overhead? Not a factor here nor was there anything else running. Just so you know also, I have an external USB C SSD that does not skip a beat in huge file transfer speeds while using apps that encode and monopolize CPU cycles. 140 is dirt slow for what these connections ought to be able to do. Migration Assistant checks for conflicts at the beginning of the transfer. When its simply copying data it should travel at near link speeds. Its basically a transposed copy window. Again, I saw consistent 450mb/sec+ on slower interface hardware (TB2) with the same exact data less than 2 weeks prior.

Heres the image:

 

[RumorConsumer]

I’m going to run a test in a while on a target disk mode’d 15” 2018 MBP to the 2017 3.5 13” to see what my transfer speeds are in a raw file copy and a Blackmagic speed test.

 

[RumorConsumer]

I just noticed my bug report has become classified as a duplicate which means Im not the only person to have reported this using Apple's developer bug report entry.