perf: optimize virtio-blk queue size and memory barrier#249
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- Increase default virtio-blk queue size from 16 to 256, matching the Linux virtio_blk default. A larger queue allows more in-flight requests and reduces per-request notification overhead. - Relax the memory barrier in VirtQueue::add() from SeqCst to Release. The fence only needs to order descriptor table and available ring writes before the available index store. A Release fence is sufficient for this producer-ordering requirement, matching Linux's dma_wmb()/virtio_wmb() semantics. On RISC-V this changes the instruction from `fence rw,rw` to `fence rw,w`. Measured on StarryOS (riscv64, QEMU TCG, 10MB sequential I/O): READ 4K: 35.74 → 42.09 MB/s (+17.8%) WRITE 4K: 1.11 → 1.15 MB/s (+3.6%)
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Two performance improvements for the virtio-blk driver.
Benchmark on StarryOS riscv64 QEMU TCG 10MB sequential IO:
READ 4K: 35.74 to 42.09 MB/s (+17.8%)
WRITE 4K: 1.11 to 1.15 MB/s (+3.6%)
Test plan: cargo check passes, StarryOS smoke test no regression.