low-latency wi-fi

part of macgamingfixes. the same article is in the app, next to the switch.

the mac's wi-fi chip may hold packets for up to 25 ms to save power. it stops when a socket asks for real-time service. wine never asks, so this fix asks for it.

symptom

on wi-fi, a quarter to half of the game's packets arrive 10–25 ms late even one hop from the router. the slow mode comes and goes on a scale of tens of seconds.

cause

the chip's power saving holds traffic up to a hard ceiling of ~25 ms unless some socket on the interface carries a real-time service class (SO_NET_SERVICE_TYPE). the ceiling is the same from 16 to 256 packets per second. the effect is interface-wide: one tagged socket speeds up every flow.

measurement

a 64 hz udp flow to the router, on ac power, 5 ghz, six alternating rounds per class.

the same probe from a winsock socket inside a test bottle: stock 10.5–14.4 ms median with 20–45 % late, fixed 4.0–4.4 ms with 0–10 %.

the fix

wine creates every windows socket inside wineserver, so one hook covers all games in the bottle. the injected library interposes socket() and tags each udp socket as interactive video.

static int my_socket(int domain, int type, int protocol) {
    int fd = socket(domain, type, protocol);
    if (fd >= 0 && udp_service && type == SOCK_DGRAM)
        setsockopt(fd, SOL_SOCKET, SO_NET_SERVICE_TYPE,
                   &udp_service, sizeof udp_service);
    return fd;
}

what changes on disk

• the same wrapper and library as registry save deferral: bin/wineserver, bin/wineserver.real, bin/wineserver_fix.dylib
• with both network fixes off, the original bin/wineserver is put back and the rest is removed

limits

• fixes the hold-off only. packet loss at the isp and bufferbloat on the router are untouched.
• on battery the same link measured p99 118 ms against 25 ms on ac. play plugged in.
• whether native games set the class themselves was not checked.
• udp service class in settings switches between interactive video and voice.