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checksum: Stream load into four registers at a time with > 128 bytes

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...and further interleave register usage. This brings the csum()
overhead reported by perf(1) for 30 seconds of 64KiB TCP IPv4
frames, host to guest, from 7.2% to 5.8%.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
Stefano Brivio 2021-10-15 17:04:46 +02:00
parent 74f29d3148
commit 4b12cf94f0
1 changed files with 47 additions and 3 deletions
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50
checksum.c
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@ -156,6 +156,8 @@ void csum_tcp4(struct iphdr *iph)
* UDP headers, and that the data is not used immediately afterwards, reducing
* cache pollution significantly and latency (e.g. on Intel Skylake: 0 instead
* of 7)
* - read from four streams in parallel as long as we have more than 128 bytes,
* not just two
* - replace the ADCQ implementation for the portion remaining after the
* checksum computation for 128-byte blocks by a load/unpack/add loop on a
* single stream, and do the rest with a for loop, auto-vectorisation seems to
@ -165,13 +167,15 @@ void csum_tcp4(struct iphdr *iph)
*/
static uint32_t csum_avx2(const void *buf, size_t len, uint32_t init)
{
__m256i zero, a, b, sum256, sum_a_hi, sum_a_lo, sum_b_hi, sum_b_lo;
__m256i a, b, sum256, sum_a_hi, sum_a_lo, sum_b_hi, sum_b_lo, c, d;
__m256i __sum_a_hi, __sum_a_lo, __sum_b_hi, __sum_b_lo;
const uint64_t *buf64 = (const uint64_t *)buf;
const __m256i *buf256 = (__m256i *)buf64;
const __m256i *buf256;
const uint16_t *buf16;
uint64_t sum64 = init;
int odd = len & 1;
__m128i sum128;
__m256i zero;
zero = _mm256_setzero_si256();
buf256 = (const __m256i *)buf64;
@ -179,7 +183,6 @@ static uint32_t csum_avx2(const void *buf, size_t len, uint32_t init)
if (len < sizeof(__m256i) * 4)
goto less_than_128_bytes;
/* We parallelize two ymm streams to minimize register dependency:
*
* a: buf256, buf256 + 2, ...
@ -203,6 +206,47 @@ static uint32_t csum_avx2(const void *buf, size_t len, uint32_t init)
len -= sizeof(__m256i) * 2;
buf256 += 2;
/* As long as we have more than 128 bytes, (stream) load from four
* streams instead of two, interleaving loads and register usage, to
* further decrease stalls, but don't double the number of accumulators
* and don't make this a general case to keep branching reasonable.
*/
if (len >= sizeof(a) * 4) {
a = _mm256_stream_load_si256(buf256);
b = _mm256_stream_load_si256(buf256 + 1);
c = _mm256_stream_load_si256(buf256 + 2);
d = _mm256_stream_load_si256(buf256 + 3);
}
for (; len >= sizeof(a) * 4; len -= sizeof(a) * 4, buf256 += 4) {
__sum_a_hi = _mm256_add_epi64(sum_a_hi,
_mm256_unpackhi_epi32(a, zero));
__sum_b_hi = _mm256_add_epi64(sum_b_hi,
_mm256_unpackhi_epi32(b, zero));
__sum_a_lo = _mm256_add_epi64(sum_a_lo,
_mm256_unpacklo_epi32(a, zero));
__sum_b_lo = _mm256_add_epi64(sum_b_lo,
_mm256_unpacklo_epi32(b, zero));
if (len >= sizeof(a) * 8) {
a = _mm256_stream_load_si256(buf256 + 4);
b = _mm256_stream_load_si256(buf256 + 5);
}
sum_a_hi = _mm256_add_epi64(__sum_a_hi,
_mm256_unpackhi_epi32(c, zero));
sum_b_hi = _mm256_add_epi64(__sum_b_hi,
_mm256_unpackhi_epi32(d, zero));
sum_a_lo = _mm256_add_epi64(__sum_a_lo,
_mm256_unpacklo_epi32(c, zero));
sum_b_lo = _mm256_add_epi64(__sum_b_lo,
_mm256_unpacklo_epi32(d, zero));
if (len >= sizeof(a) * 8) {
c = _mm256_stream_load_si256(buf256 + 6);
d = _mm256_stream_load_si256(buf256 + 7);
}
}
for (; len >= sizeof(a) * 2; len -= sizeof(a) * 2, buf256 += 2) {
a = _mm256_stream_load_si256(buf256);
b = _mm256_stream_load_si256(buf256 + 1);