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
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