Re: [PATCH 9/9] qemu-file: Account for rate_limit usage on qemu_fflush()

[Juan Quintela <quintela@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> wrote:
> On Thu, May 04, 2023 at 01:38:41PM +0200, Juan Quintela wrote:
>> That is the moment we know we have transferred something.
>> 
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>> ---
>>  migration/qemu-file.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>> 
>> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
>> index ddebfac847..309b4c56f4 100644
>> --- a/migration/qemu-file.c
>> +++ b/migration/qemu-file.c
>> @@ -300,7 +300,9 @@ void qemu_fflush(QEMUFile *f)
>>                                     &local_error) < 0) {
>>              qemu_file_set_error_obj(f, -EIO, local_error);
>>          } else {
>> -            f->total_transferred += iov_size(f->iov, f->iovcnt);
>> +            uint64_t size = iov_size(f->iov, f->iovcnt);
>> +            qemu_file_acct_rate_limit(f, size);
>> +            f->total_transferred += size;
>>          }
>>  
>>          qemu_iovec_release_ram(f);
>> @@ -527,7 +529,6 @@ void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
>>          return;
>>      }
>>  
>> -    f->rate_limit_used += size;
>>      add_to_iovec(f, buf, size, may_free);
>>  }
>>  
>> @@ -545,7 +546,6 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
>>              l = size;
>>          }
>>          memcpy(f->buf + f->buf_index, buf, l);
>> -        f->rate_limit_used += l;
>>          add_buf_to_iovec(f, l);
>>          if (qemu_file_get_error(f)) {
>>              break;
>> @@ -562,7 +562,6 @@ void qemu_put_byte(QEMUFile *f, int v)
>>      }
>>  
>>      f->buf[f->buf_index] = v;
>> -    f->rate_limit_used++;
>>      add_buf_to_iovec(f, 1);
>>  }
>
> This has a slight semantic behavioural change.

Yeap.

See the answer to Peter.  But three things came to mind:

a - the size of the buffer is small (between 32KB and 256KB depending
    how you count it).  So we are going to call qemu_fflush() really
    soon.

b - We are using this value to calculate how much we can send through
    the wire.  Here we are saything how much we have accepted to send.

c - When using multifd the number of bytes that we send through the qemu
    file is even smaller. migration-test multifd test send 300MB of data
    through multifd channels and around 300KB on the qemu_file channel.


>
> By accounting for rate limit in the qemu_put functions, we ensure
> that we stop growing the iovec when rate limiting activates.
>
> If we only apply rate limit in the the flush function, that will
> let the  f->iov continue to accumulate buffers, while we have
> rate limited the actual transfer.

256KB maximum.  Our accounting has bigger errors than that.


> This makes me uneasy - it feels like a bad idea to continue to
> accumulate buffers if we're not ready to send them

I still think that the change is correct.  But as you and Peter have
concerns about it, I will think a bit more about it.

Thanks, Juan.