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authorStefano Brivio <sbrivio@redhat.com>2021-10-15 17:04:46 +0200
committerStefano Brivio <sbrivio@redhat.com>2021-10-15 17:04:46 +0200
commit4b12cf94f078dd54e2ed7a2202104ddf6f3be69a (patch)
treed3cabfbb1edcf77f6ab1557e0ec3e9f6ef6ccfad
parent74f29d3148d34d166c040e6cf1f626245c0d479a (diff)
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checksum: Stream load into four registers at a time with > 128 bytes
...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>
-rw-r--r--checksum.c50
1 files changed, 47 insertions, 3 deletions
diff --git a/checksum.c b/checksum.c
index c79c9d7..72a7bba 100644
--- a/checksum.c
+++ b/checksum.c
@@ -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);