multifd_send() and multifd_recv() are on the per-page-batch hot path of live migration. Both functions call migrate_multifd_channels() multiple times (3-4 calls each) for modulo arithmetic in the round-robin channel selection loop.
Each call goes through migrate_get_current() -> dereference MigrationState -> read parameters.multifd_channels. While each individual call is cheap, these functions execute for every page batch during the entire migration, easily millions of times. Cache the return value in a local variable at function entry. The channel count is fixed for the duration of a migration and cannot change mid-flight. For multifd_send(): 3 calls reduced to 1. For multifd_recv(): 4 calls reduced to 1. Signed-off-by: Bin Guo <[email protected]> --- migration/multifd.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/migration/multifd.c b/migration/multifd.c index 67ee9bdf5e..cc2fa90204 100644 --- a/migration/multifd.c +++ b/migration/multifd.c @@ -362,13 +362,15 @@ bool multifd_send(MultiFDSendData **send_data) /* We wait here, until at least one channel is ready */ qemu_sem_wait(&multifd_send_state->channels_ready); + int thread_count = migrate_multifd_channels(); + /* * next_channel can remain from a previous migration that was * using more channels, so ensure it doesn't overflow if the * limit is lower now. */ - next_channel %= migrate_multifd_channels(); - for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) { + next_channel %= thread_count; + for (i = next_channel;; i = (i + 1) % thread_count) { if (multifd_send_should_exit()) { return false; } @@ -378,7 +380,7 @@ bool multifd_send(MultiFDSendData **send_data) * sender thread can clear it. */ if (qatomic_read(&p->pending_job) == false) { - next_channel = (i + 1) % migrate_multifd_channels(); + next_channel = (i + 1) % thread_count; break; } } @@ -998,6 +1000,7 @@ bool multifd_recv(void) int i; static int next_recv_channel; MultiFDRecvParams *p = NULL; + int thread_count = migrate_multifd_channels(); MultiFDRecvData *data = multifd_recv_state->data; /* @@ -1005,8 +1008,8 @@ bool multifd_recv(void) * using more channels, so ensure it doesn't overflow if the * limit is lower now. */ - next_recv_channel %= migrate_multifd_channels(); - for (i = next_recv_channel;; i = (i + 1) % migrate_multifd_channels()) { + next_recv_channel %= thread_count; + for (i = next_recv_channel;; i = (i + 1) % thread_count) { if (multifd_recv_should_exit()) { return false; } @@ -1014,7 +1017,7 @@ bool multifd_recv(void) p = &multifd_recv_state->params[i]; if (qatomic_read(&p->pending_job) == false) { - next_recv_channel = (i + 1) % migrate_multifd_channels(); + next_recv_channel = (i + 1) % thread_count; break; } } -- 2.50.1 (Apple Git-155)
