Measured: M4 Max multi-display allocation behavior with 3x 5K/165 panels (sticky pipes, synthesized modes, TB vs HDMI pooling)

[bnaert]

Measured: how the M4 Max display allocator actually behaves (3× 5K/165 panels + mixed configs)

This community — and discussion #4871 in particular — is where I found the only real prior art on macOS multi-display high-refresh limits, so I wanted to bring the data back here. I spent a week probing an M4 Max Mac Studio (macOS 27 Public Beta 2) with three LG 27GM950B panels (5120×2880 @ 165Hz native, DP 2.1/HDMI 2.1, DSC), plus a 4K/240 ASUS PG32UCDM3 and two QHD/144 ProArts for controls. Summary of what the allocator does, with the receipts condensed:

Findings

1. Per-display limits are soft. A single 27GM950B negotiates full native 5K @ 165 (2.43 Gpx/s) over both HDMI 2.1 and TB5 → USB-C-to-DP 2.1 cable — the monitor OSD confirms a genuine DP 2.1 link on the TB path. Apple documents 5K/120 as the single-display max; DSC makes the extra headroom real.

2. Two fast pipes, both claimable. Two GM9s run 5K/165 simultaneously (~4.87 Gpx/s) — one HDMI, one TB→DP. Apple's docs imply only one "fast class" display in 3+ display configs; in practice both fast pipes stay grantable.

3. Assignment is sticky and happens at enumeration. Once displays are connected, nothing re-deals the pipes. Concrete proof: in a triple config with the third display capped at 5K/60, manually dropping all three displays to 60Hz (2.65 Gpx/s total — headroom everywhere) still leaves the third display's mode list topping out at 60. Freeing bandwidth after the fact is invisible. Only physical replug re-enumerates.

4. Three distinct over-budget behaviors, never renegotiation:

• Refusal: with dual-165 held, a third GM9 on any TB port refuses to enumerate — no signal, no fallback mode.
• Basement demotion: hot-plugging into a marginally-over mix (2× GM9 + 2× QHD) drops the newcomer to a 3200×1800 @ 60 mode (~0.35 Gpx/s).
• Tailored synthesis: with 165/165 held and a 4K/240 panel plugged last on HDMI, macOS offers a fixed, synthesized 4K @ 97Hz as the top of the mode list (97/60/50/30/25/24). No standard timing exists at 97; it's computed.

5. The 97Hz mode is a continuous probe of the budget. 4K@97 ≈ 0.805 Gpx/s; add 2× 2.433 and the machine total is ~5.67 Gpx/s. It also retro-explains the GM9's 5K/60 grant on the same seat: 0.885 fits, 5K/65 (0.96) wouldn't — 60 was the largest EDID mode fitting the remainder, same algorithm. Working model: two priority fast pipes drawing first from a ~5.7 Gpx/s machine budget, third seat sized to the remainder.

6. Pooling by output path. With both fast grants on Thunderbolt, a third display on any TB port gets nothing, while the same panel on HDMI comes up at the remainder mode. The HDMI output behaves like an independent pipe — in the triple config it's the reason a third display exists at all.

7. Workaround that works without EDID tools: pre-setting the connected displays' refresh before hot-plugging the next display changes what the allocator grants (120/120 pre-set → third display enumerates instead of refusing). This is how the first triple config came up.

8. No Adaptive-Sync anywhere. None of these panels (full G-Sync/FreeSync on Windows) ever show a variable option on macOS in these configs — presumably the usual DP-only / no-HDMI-VRR / scaled+DSC interference constraints.

Full pixel-rate ledger

Config	Aggregate (Gpx/s)	Outcome
1× 5K165	2.43	✅ either port
2× 5K165	4.87	✅
2× 5K165 + 2× QHD144	5.92 req.	❌ newcomer demoted to 0.35
3× 5K165	7.30	❌ third refuses to enumerate
2× 5K120 + 5K60	4.42	✅ (via pre-set workaround)
2× 5K165 + 5K60	5.75	✅ best all-5K result
2× 5K165 + 4K@97 (synthesized)	5.67	✅ best mixed result
3× on Thunderbolt, any refresh	—	❌ no signal; third stream requires HDMI

(For contrast: the same three panels on an RTX 5090 run 165/165/165 at 10-bit — the limits above are allocator behavior, not the panels or links.)

Questions for people who know the internals

1. Does anything in IOKit / the display pipeline corroborate the "two fast pipes + remainder seat + ~5.7 Gpx/s budget" model, or is there a better framing?
2. Is there a reason the HDMI path acts as an independent pipe while the TB side refuses a third stream outright?
3. The BetterDisplay-shaped question: since assignment is decided at enumeration, could a pre-connection EDID override (presenting a panel as, say, 6K/60-class) coax the TB side into granting a third TB stream? Sticky assignment means the override must exist before connect — is that feasible with BetterDisplay's EDID tooling today?
4. Remaining discriminating experiment I can run: whether 2× QHD/144 at plain native (no supersampling) coexist with 165/165/60 — pipe model says yes (~each fits a standard slot), pure aggregate budget says no (~6.8 total). Happy to test and report back, along with any other configs people want probed.

Full writeup with the step-by-step receipts: https://gist.github.com/bnaert/14a4ad2e3ab5ece3a3520fe44a928fb9