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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 9 Jan 2014 15:25:29 +0200

On Thu, Jan 09, 2014 at 12:28:50AM -0800, Michael Dalton wrote:
> 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.

I really think it's best to start with 256 alignment.
A bit simpler, better than what we had, and will let us go
above PAGE_SIZE long term.  The optimization shrinking
alignment to 64 can be done on top if we see a
work-load that's improved by it, which I doubt.

> 
> #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);

You can just do len * MERGEABLE_BUFFER_ALIGN too - my compiler seems
to be smart enough to see it's same.
This way there's no need for 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);

uintptr_t? or just unsigned long ...

> }
> /* 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));
> }

The APIs are a bit inconsistent in naming.
Also it's unfortunate that we use void * for both virtio buffer
and frame memory.

How about
	mergeable_buf_to_len
	mergeable_buf_to_page
	mergeable_buf_to_offset
	mergeable_page_to_buf

this way we don't use void * for frame memory.

alternatively, cast virtio buffer to unsigned long
immediately and pass that around everywhere.

> /* 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