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2013 Mac Pro: Thunderbolt Performance Tips

2013 Apple Mac Pro rear ports

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Thunderbolt in the 2013 Mac Pro provides a versatile expansion capability, but it comes with a complexity that is essential to understand, or performance can be lost.

Understanding the port layout and the three Thunderbolt busses is essential, because where a device plugs in can matter when other devices are also on the same bus.

Three Thunderbolt busses

Click the image at right for larger view.

The 2013 Mac Pro has three (3) Thunderbolt busses irregularly arranged not visibly labeled or numbered by Apple, for maximum user confusion:

  • Bus #0: two ports plus the HDMI port.
    (Bottom two ports plus HDMI port)
  • Bus #1: the two ports at top/left.
  • Bus #2: the two ports at top/right.

Each Thunderbolt bus has dedicated bandwidth not shared by the others.

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Consider USB3 for some purposes

The Mac Pro also has four USB3 ports (effectively three, given the use of one for a keyboard and mouse), which are quite capable in performance terms, and thus are useful for offloading lower performance devices for which USB3 provides adequate bandwidth.

For example, the OWC Mercury Elite Dual offers a choice of Thunderbolt or USB3 connectivity; performance is nearly identical with either, so USB3 is a perfectly fine choice. In general, a dual hard drive unit has ample bandwidth with USB3. Units with three or more drives should generally perform better via Thunderbolt. Still, USB3 can deliver around ~430 MB/sec using an SSD, and each USB3 port should be able to deliver that bandwidth (however, not tested and confirmed as yet but 800+ MB/sec over two ports confirmed on MacBook Pro). Not as fast as Thunderbolt per port, but an excellent alternative for small fast SSDs that are bus powered.

General performance notes

Performance is ample for most system configurations, especially for devices like hard drives. The issue comes when utilizing high performance devices (e.g. fast SSDs) particularly a device that by itself uses most of the Thunderbolt v1 bandwidth (~825 MB/sec).

When attaching Thunderbolt devices, these general principles should be followed to maximize performance:

  • Daisy chaining is perfectly fine for lower performance devices and/or for devices that as per one’s own usage patterns are not used simultaneously (e.g., bandwidth is not in demand by multiple devices at the same time).
  • Avoid daisy-chaining high performance devices.
  • Spread the load” (specific to one’s own workflow): consider both peak device bandwidth requirements as well as simultaneous or non-simultaneous utilization. Two very high performance devices can work at peak speed on the same bus if not used simultaneously (no actual competition for bandwidth).
  • Attach ultra high performance Thunderbolt 2 devices on separate busses when feasible. There are only two choices of bus, since the display(s) scarfs the bandwidth on one bus.
  • Prefer Thunderbolt v2 for high performance devices (few and far between as of early 2014).
  • Perspective should be kept in mind: all four internal SATA ports in the 2010/2013 Mac Pro are maxed out in total at around 650 MB/sec.
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The design challenge

Consider your author’s setup (TB1 = Thunderbolt, TB2 = Thunderbolt 2). But none of these devices are Thunderbolt 2.

Several of these devices can each consume 80% to 90% of Thunderbolt v1 bandwidth when in full use. Hence the system design has to take into account the “spread the load” principle: attaching the devices in such a way that each can run at full speed. Or at least, to not attach in a way that typical usage will degrade the performance of one device.

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Example as tested

The graph below shows three test scenarios using two high performance Thunderbolt v1 devices:

  • Each device tested while the other is idle (“one device idle”). This shows the peak performance of each doing disk reads.
  • Tested simultaneously using separate ports on the same Thunderbolt bus.
  • Tested simultaneously but daisy chained (Thunderbay first in the chain).

The results show unequivocally that daisy-chaining two high speed devices can result in a considerable loss of performance: the Accelsior SSD is throttled to 250 MB/sec, down from 701 MB/sec! The best solution here is to connect each device to a different bus, or at least plug each device in directly (no daisy chaining).

Effect of daisy-chaining two high-performance Thunderbolt v1 devices vs separate ports

As an example, consider a clone backup to be made off the Master volume (a fast SSD) onto a triple RAID-1 mirror backup set*. Assuming 160 MB/sec hard drive speed, the backup mirror set requires 480MB/sec bandwidth. If the original SSD is external and on the same bus, then the combined bandwidth is 960MB/sec, which exceeds what Thunderbolt v1 can deliver; performance will decline. Hence the backup device and the source device should ideally not be on the same bus. Moreover, other work is likely in progress while the backup proceeds, which might place additional demands on bandwidth (e.g. saving a 14GB Photoshop file). Fortunately, such other work can be done and is best done on the fast internal Mac Pro SSD.

* Your author regularly makes clone backups to triple-drive RAID-1 mirrors, for redundancy and efficiency.

2013 Apple Mac Pro rear ports
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