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101 lines
5.4 KiB
Markdown
101 lines
5.4 KiB
Markdown
# The client's create window (MOBILEGL_IPC_CREATE_WINDOW)
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Measured 2026-09-26 on the split stack (client and server in separate processes). These are the numbers
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that used to live in the comment above `IpcTable::CreateWindow` in `MobileGL/Config.h`. The longer record,
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with the logs and the attempts that failed, is in the git history (in Chinese).
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## What the knob does
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Every new buffer, texture or renderbuffer sends a `resource_create` record. When these numbers were taken,
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the client waited for the server's answer to every one of them, because a create the server refused must be
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un-published so that later records for that object take the blocking path.
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With a window of N, up to N creates go out without waiting. Each caller reads a provisional accept, the
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answers are collected in order, and the oldest is waited for once the window is full. A create that comes
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back declined is un-published when its answer is collected. 1 turns the window off.
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The window runs only on a link that keeps a declared answer until its reader takes it
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(`ILink`'s `RetainsReplies`, the stream link's declared-answer store). On the shared-memory link it is
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always 1.
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## Setup
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- Server: an Android phone (a generic system image on a MediaTek mt6893 with a Mali GPU, Android 14).
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- Client: Minecraft on a separate machine, connected over TCP with the stream data plane.
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- Workload: the single long frame in which the game loads its world. Every arm sends the same records:
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4,536 creates, 103,097 records in all and about 101 MB of staged bytes, with identical per-op counts.
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- "Blocking waits" counts the times the client blocked on a create's answer, on the blocking path or while
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collecting the oldest answer from a full window (`by_op 2` in the client's link metrics). "Wall clock" is
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the duration of that frame.
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## Why the window did nothing at first
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The first version changed nothing: window 4 and window 1 both took 4,536 blocking waits and 28.64 s. It
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took three findings, in this order, to make it work:
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1. The client kept answers in a ring of the last 8. Between two creates the client emits on the order of
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a thousand records, so the answer was always gone by the time the client came back for it (at the first
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miss the head wanted seq 310 while the ring held seqs 1,175 to 1,504, and it wanted 310 on every miss
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after that).
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2. A residency budget that tried to collect answers early could not help: what displaces an answer is
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what the server sends, not what the client emits, and the server is behind.
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3. So the fix was retention: a link keeps a declared answer until its reader takes it.
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The answer is kept because the wire declared that it will be read, not because the caller did not wait.
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The window's own creates are both fire-and-forget and read, so a flag keyed on "the caller does not wait"
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would switch off the very mechanism it exists to enable.
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## Results
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### With retention, both ends rebuilt
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| window | blocking waits | wall clock | round trip per blocking wait |
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|---:|---:|---:|---:|
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| 1 (the original blocking shape) | 4,536 | 28.66 s | 5,672 us |
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| 2 | 2,500 | 17.27 s | 5,285 us |
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| 4 | 1,644 | 11.02 s | 4,082 us |
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The round trip does not grow as the window deepens. The roughly 5 ms is a wakeup cost that the server
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batches away, not its apply throughput, so collecting four creates per wakeup is a real 2.6x.
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A first sweep with three interleaved runs per arm showed the curve still falling at the old ceiling of 4
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(1 to 2 saved 10.29 s, 2 to 3 saved 3.74 s, 3 to 4 saved 1.83 s), so the ceiling was raised.
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### Sweep past 4 (one binary, three interleaved runs per arm)
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| window | blocking waits | wall clock, median | spread |
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|---:|---:|---:|---:|
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| 4 | 1,503 | 13.23 s | 11.37 - 16.29 s |
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| 5 | 1,277 | 11.31 s | 10.61 - 12.12 s |
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| 6 | 1,155 | 10.79 s | 9.69 - 10.81 s |
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| 8 | 859 | 10.96 s | 8.92 - 11.03 s |
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## Reading
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- 4 to 5 saves 1.92 s, 5 to 6 saves 0.53 s, and 6 to 8 saves nothing (-0.18 s, inside the session noise
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of about 1.5 to 2 s). The curve is flat from about 6.
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- Past that point the frame is not limited by the window: window 8 has 296 fewer blocking waits than
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window 6 and the wall clock does not move.
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- The answer store is not the limit either. Its peak, logged by the stream link, read 6 to 11 entries
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against a capacity of 24 at every window from 5 up.
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- What remains is a roughly constant 350 to 400 creates that block for reasons the window depth does not
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reach, and the 101 MB and 103k records the server has to apply. A later server-side timing at window 6
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put the server's apply time at 1.95 s of a 9.64 s frame, while the client's 1,161 blocking waits at
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about 4.9 ms each came to 5.7 s. Sending the same bytes over USB instead of Wi-Fi did not change the
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wall clock.
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## What shipped
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| commit | default | ceiling |
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|---|---:|---:|
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| `3a255580` (retention lands) | 2 | 4 |
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| `102f2ca7` (sweep above) | 6 | 8 |
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| `3a6c5c7f` (current) | 10 | 12 |
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No sweep beyond 8 is recorded. 10 sits on the flat part of the curve, and a deeper window costs only
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entries in the stream link's answer store. The ceiling, `kCreateWindowMax` in
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`MobileGL/MG_Remote/Client/WireTables.cpp`, is held by a `static_assert` to at most half of that store's
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24 entries; the other half is for the one blocking create in flight and for answers a retired window has
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left behind. The store's overflow is a named fatal error, never an eviction. The environment variable
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accepts 1 to 12; a value outside that range is ignored with a warning and the default is used.
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