fix(rdma): default pipeline depth 1 to 4 for server and client

[ketor]

Problem

On GLM-5.2-NVFP4 (DSA multi-pool model) with dfkv L3, the hot round throughput is -29.8% worse than cold:

Metric	Cold	Hot	Change
Total throughput	37,306 tok/s	26,191 tok/s	-29.8%
Median TTFT	15.1s	32.4s	+114%
Mean TPOT	228ms	139ms	-39% (improved)
Prefetch failures	0	532	---
PUT batch failures	---	332	---

Root Cause

Server and client RDMA pipeline depth default=1, production uses depth=4.

The dfkv server defaults DFKV_RDMA_DEPTH to 1 (rdma_server.cc). The client also defaults to 1 (rdma_transport.cc). All production deployments (bj09, xb01, hd04) set DFKV_RDMA_DEPTH=4 explicitly. When the default is used, the client/server depth mismatch causes:

1. Pipelined GET degradation (3-4x, per existing code comment in rdma_server.h)
2. PUT batch failures during burst writes: SGLang backup writes 128 pages per batch. With depth=1, the single in-flight slot creates backpressure, entire 128-page batches fail, pages never written to L3
3. Hot-round prefetch failures: missing pages cause batch_get_v1 to return False, prefetch terminates, falls back to cold compute

Fix

Change both defaults from 1 to 4, matching the standard production value:

• rdma_server.cc:291: server default depth 1 to 4
• rdma_transport.cc:231: client default depth 1 to 4
• Update help text and comments

No protocol change, no wire format change. Explicit --rdma-depth or DFKV_RDMA_DEPTH env var still overrides.

Verification (xb01-0064, 8xB200, GLM-5.2-NVFP4, dfkv v2.6.3)

Before fix (server depth=1, client depth=4)

Metric	Cold	Hot	Hot vs Cold
Total throughput	37,306 tok/s	26,191 tok/s	-29.8%
Median TTFT	15,139 ms	32,448 ms	+114%
Prefetch failures	0	532	---
Depth clamp warnings	>0	---	---

After fix (server depth=4, client depth=4)

Metric	Cold	Hot	Hot vs Cold
Total throughput	38,549 tok/s	37,409 tok/s	-2.9%
Median TTFT	13,940 ms	14,990 ms	+7.2%
Prefetch failures	0	0	---
Depth clamp warnings	0	0	---

Improvement

• Hot vs cold degradation: -29.8% to -2.9% (90% reduction)
• Prefetch failures: 532 to 0
• Hot throughput: +42.9% (26,191 to 37,409 tok/s)

Files Changed

• src/cache/rdma_server.cc: ServerDepth() default 1 to 4
• src/cache/rdma_server.h: header comment update
• src/cache/dfkv_server_main.cc: help text update
• src/transport/rdma_transport.cc: RdmaTransport constructor depth_ default 1 to 4
• src/transport/rdma_transport.h: header comment update

Scope

• Changes defaults only; explicit env/flag still overrides
• No protocol change, no wire format change
• Existing deployments with explicit DFKV_RDMA_DEPTH=4 are unaffected
• Only affects deployments that relied on the default (depth=1)