netdev - Re: [PATCH net-next v2 3/4] virtio-net: auto-tune mergeable rx buffer size for improved performance

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Date:	Thu, 9 Jan 2014 00:28:50 -0800
From:	Michael Dalton <mwdalton@...gle.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jason Wang <jasowang@...hat.com>,
	lf-virt <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH net-next v2 3/4] virtio-net: auto-tune mergeable rx buffer
 size for improved performance

Hi Michael,

Here's a quick sketch of some code that enforces a minimum buffer
alignment of only 64, and has a maximum theoretical buffer size of
aligned GOOD_PACKET_LEN + (BUF_ALIGN - 1) * BUF_ALIGN, which is at least
1536 + 63 * 64 = 5568. On x86, we already use a 64 byte alignment, and
this code supports all current buffer sizes, from 1536 to PAGE_SIZE.

#if L1_CACHE_BYTES < 64
#define MERGEABLE_BUFFER_ALIGN 64
#define MERGEABLE_BUFFER_SHIFT 6
#else
#define MERGEABLE_BUFFER_ALIGN L1_CACHE_BYTES
#define MERGEABLE_BUFFER_SHIFT L1_CACHE_SHIFT
#endif
#define MERGEABLE_BUFFER_MIN ALIGN(GOOD_PACKET_LEN +
                                   sizeof(virtio_net_hdr_mrg_rbuf),
                                   MERGEABLE_BUFFER_ALIGN)
#define MERGEABLE_BUFFER_MAX min(MERGEABLE_BUFFER_MIN +
                                 (MERGEABLE_BUFFER_ALIGN - 1) *
                                 MERGEABLE_BUFFER_ALIGN, PAGE_SIZE)
/* Extract buffer length from a mergeable buffer context. */
static u16 get_mergeable_buf_ctx_len(void *ctx) {
        u16 len = (uintptr_t)ctx & (MERGEABLE_BUFFER_ALIGN - 1);
        return MERGEABLE_BUFFER_MIN + (len << MERGEABLE_BUFFER_SHIFT);
}
/* Extract buffer base address from a mergeable buffer context. */
static void *get_mergeable_buf_ctx_base(void *ctx) {
        return (void *) ((uintptr)ctx & -MERGEABLE_BUFFER_ALIGN);
}
/* Convert a base address and length to a mergeable buffer context. */
static void *to_mergeable_buf_ctx(void *base, u16 len) {
        len -= MERGEABLE_BUFFER_MIN;
        return (void *) ((uintptr)base | (len >> MERGEABLE_BUFFER_SHIFT));
}
/* Compute the packet buffer length for a receive queue. */
static u16 get_mergeable_buffer_len(struct receive_queue *rq) {
        u16 len = clamp_t(u16, MERGEABLE_BUFFER_MIN,
                          ewma_read(&rq->avg_pkt_len),
                          MERGEABLE_BUFFER_MAX);
        return ALIGN(len, MERGEABLE_BUFFER_ALIGN);
}

Best,

Mike
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