Re: [Qemu-devel] [PATCH] migration: fix rate limiting

[Paolo Bonzini]

Il 20/11/2012 12:03, Juan Quintela ha scritto:
> This patch is wrong O;-)
> That don't mean that current code is right.
> 
> We have 4 variables:
> - xfer_limit: how much we are allowed to send each 100ms (in bytes)
> - buffer_capacity: the size of the buffer that we are using
> - buffer_size: the amount of the previous buffer that we are using
>                buffer_size < buffer_capacity, or we are doing something
>                 wrong.
> - bytes_xfer: How many bytes we have transfered since last 100ms timer.

Note that the buffered_file places a wall between producer and consumer
(or rather tries to place it; my patch is an attempt to fix).  The
consumer side is buffered_flush & bytes_xfer, and the rate-limiting
there is mandatory.

However, on the producer side, the rate-limiting is only advisory, to
avoid making the buffer_capacity too large.  In principle (and leaving
MAX_WAIT aside for a moment) you could skip qemu_file_rate_limit calls
completely.  It would place the whole RAM in the buffer, and still
transfer it in xfer_limit chunks on the wire.

> And how this work:
> - we have an input handler that copies stuff from RAM to buffer
>   it stops when we have sent more that xfer_limit on this period (each
>   100ms)
> - we have another handler that is run each 100ms, and this one sent
>   anything that is on the buffer, and reset bytes_xfer to zero.
> 
> WHat you have done is just telling that in the 1st input handler, that
> we always have size on the buffer for it, so that we are not doing any
> rate limiting at all.
> 
> It is very strange that buffer_capacity is bigger that xfer_limit (it
> could happen, but it is very unusual), and then what you have done there
> is just disabling rate limiting altogether.

I haven't: once s->bytes_xfer >= s->xfer_limit, buffered_flush will not
do anything, and data will accumulate in the buffer until bytes_xfer is
moved back to 0.  So what happens really is that ram_save_block is
already preparing the next chunk of data to send, but only s->xfer_limit
bytes are sent on each 100ms period.

However, my patch was incomplete.  The desired behavior is that
buffered_put_buffer(s, NULL, 0, 0) will restart the iteration, so it has
to check buffer_size too.  In fact, the check in buffered_put_b

There is also another bug in the current code, which is an off-by-one.
Comparison is s->bytes_xfer > s->xfer_limit, but it should be >= instead.

And more somewhat broken checks.  I'm testing a more complete fix.

Paolo

> 
> So ....
> 
> NACK
> 
>>
>> Signed-off-by: Paolo Bonzini <address@hidden>
>> ---
>>  buffered_file.c | 2 +-
>>  1 file modificato, 1 inserzione(+). 1 rimozione(-)
>>
>> diff --git a/buffered_file.c b/buffered_file.c
>> index bd0f61d..46cd591 100644
>> --- a/buffered_file.c
>> +++ b/buffered_file.c
>> @@ -193,7 +193,7 @@ static int buffered_rate_limit(void *opaque)
>>      if (s->freeze_output)
>>          return 1;
>>  
>> -    if (s->bytes_xfer > s->xfer_limit)
> o> +    if (s->buffer_size > s->xfer_limit)
>>          return 1;
>>  
>>      return 0;