Re: [Qemu-devel] [PATCH v2 3/8] multifd: Create new next_packet_size field

[Juan Quintela]

"Dr. David Alan Gilbert" <address@hidden> wrote:
> * Juan Quintela (address@hidden) wrote:
>> We need to send this field when we add compression support.  As we are
>> still on x- stage, we can do this kind of changes.
>
> Can you explain this a bit more; I'm confused how you can know what the
> next size is going to be until you've got there.

This is needed for compression.  Without compression, we always know the
size of packet: number of pages * page size + some headers (that don't
matter here).

With compression that changes, and it is *much* easier for zlib if we
read the full packet in advance to a buffer and then tell zlib to
uncompress it.  It is possible to do it otherwise, but it is a mess (not
that zlib is not *always* a mess).

So with this change, I  can do a:
- read header (it is not compressed and has the size of the following
part of the packet)
- read rest of the packet in one go
- uncompress it like a champ, without worrying that we need to still
read something else.

Later, Juan.


> Dave
>
>> Signed-off-by: Juan Quintela <address@hidden>
>> ---
>>  migration/ram.c        | 15 +++++++++++++--
>>  migration/trace-events |  4 ++--
>>  2 files changed, 15 insertions(+), 4 deletions(-)
>> 
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 29f0d431a8..26ed26fc2d 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -534,6 +534,8 @@ typedef struct {
>>      /* maximum number of allocated pages */
>>      uint32_t pages_alloc;
>>      uint32_t pages_used;
>> +    /* size of the next packet that contains pages */
>> +    uint32_t next_packet_size;
>>      uint64_t packet_num;
>>      char ramblock[256];
>>      uint64_t offset[];
>> @@ -581,6 +583,8 @@ typedef struct {
>>      MultiFDPacket_t *packet;
>>      /* multifd flags for each packet */
>>      uint32_t flags;
>> +    /* size of the next packet that contains pages */
>> +    uint32_t next_packet_size;
>>      /* global number of generated multifd packets */
>>      uint64_t packet_num;
>>      /* thread local variables */
>> @@ -617,6 +621,8 @@ typedef struct {
>>      /* global number of generated multifd packets */
>>      uint64_t packet_num;
>>      /* thread local variables */
>> +    /* size of the next packet that contains pages */
>> +    uint32_t next_packet_size;
>>      /* packets sent through this channel */
>>      uint64_t num_packets;
>>      /* pages sent through this channel */
>> @@ -721,6 +727,7 @@ static void multifd_send_fill_packet(MultiFDSendParams 
>> *p)
>>      packet->flags = cpu_to_be32(p->flags);
>>      packet->pages_alloc = cpu_to_be32(migrate_multifd_page_count());
>>      packet->pages_used = cpu_to_be32(p->pages->used);
>> +    packet->next_packet_size = cpu_to_be32(p->next_packet_size);
>>      packet->packet_num = cpu_to_be64(p->packet_num);
>>  
>>      if (p->pages->block) {
>> @@ -772,6 +779,7 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams 
>> *p, Error **errp)
>>          return -1;
>>      }
>>  
>> +    p->next_packet_size = be32_to_cpu(packet->next_packet_size);
>>      p->packet_num = be64_to_cpu(packet->packet_num);
>>  
>>      if (p->pages->used) {
>> @@ -1011,6 +1019,7 @@ static void *multifd_send_thread(void *opaque)
>>              uint64_t packet_num = p->packet_num;
>>              uint32_t flags = p->flags;
>>  
>> +            p->next_packet_size = used * qemu_target_page_size();
>>              multifd_send_fill_packet(p);
>>              p->flags = 0;
>>              p->num_packets++;
>> @@ -1018,7 +1027,8 @@ static void *multifd_send_thread(void *opaque)
>>              p->pages->used = 0;
>>              qemu_mutex_unlock(&p->mutex);
>>  
>> -            trace_multifd_send(p->id, packet_num, used, flags);
>> +            trace_multifd_send(p->id, packet_num, used, flags,
>> +                               p->next_packet_size);
>>  
>>              ret = qio_channel_write_all(p->c, (void *)p->packet,
>>                                          p->packet_len, &local_err);
>> @@ -1253,7 +1263,8 @@ static void *multifd_recv_thread(void *opaque)
>>  
>>          used = p->pages->used;
>>          flags = p->flags;
>> -        trace_multifd_recv(p->id, p->packet_num, used, flags);
>> +        trace_multifd_recv(p->id, p->packet_num, used, flags,
>> +                           p->next_packet_size);
>>          p->num_packets++;
>>          p->num_pages += used;
>>          qemu_mutex_unlock(&p->mutex);
>> diff --git a/migration/trace-events b/migration/trace-events
>> index bd2d0cd25a..a11e66e1d9 100644
>> --- a/migration/trace-events
>> +++ b/migration/trace-events
>> @@ -77,13 +77,13 @@ get_queued_page_not_dirty(const char *block_name, 
>> uint64_t tmp_offset, unsigned
>>  migration_bitmap_sync_start(void) ""
>>  migration_bitmap_sync_end(uint64_t dirty_pages) "dirty_pages %" PRIu64
>>  migration_throttle(void) ""
>> -multifd_recv(uint8_t id, uint64_t packet_num, uint32_t used, uint32_t 
>> flags) "channel %d packet number %" PRIu64 " pages %d flags 0x%x"
>> +multifd_recv(uint8_t id, uint64_t packet_num, uint32_t used, uint32_t 
>> flags, uint32_t next_packet_size) "channel %d packet number %" PRIu64 " 
>> pages %d flags 0x%x next packet size %d"
>>  multifd_recv_sync_main(long packet_num) "packet num %ld"
>>  multifd_recv_sync_main_signal(uint8_t id) "channel %d"
>>  multifd_recv_sync_main_wait(uint8_t id) "channel %d"
>>  multifd_recv_thread_end(uint8_t id, uint64_t packets, uint64_t pages) 
>> "channel %d packets %" PRIu64 " pages %" PRIu64
>>  multifd_recv_thread_start(uint8_t id) "%d"
>> -multifd_send(uint8_t id, uint64_t packet_num, uint32_t used, uint32_t 
>> flags) "channel %d packet_num %" PRIu64 " pages %d flags 0x%x"
>> +multifd_send(uint8_t id, uint64_t packet_num, uint32_t used, uint32_t 
>> flags, uint32_t next_packet_size) "channel %d packet_num %" PRIu64 " pages 
>> %d flags 0x%x next packet size %d"
>>  multifd_send_sync_main(long packet_num) "packet num %ld"
>>  multifd_send_sync_main_signal(uint8_t id) "channel %d"
>>  multifd_send_sync_main_wait(uint8_t id) "channel %d"
>> -- 
>> 2.20.1
>> 
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK