[libvirt] [PATCH 1/2] rpc: Switch to dynamically allocated message buffer

[Michal Privoznik]

On 03.05.2012 10:01, Daniel P. Berrange wrote:
> On Wed, May 02, 2012 at 01:52:27PM -0600, Eric Blake wrote:
>> On 04/27/2012 07:22 AM, Michal Privoznik wrote:
>>> Currently, we are allocating buffer for RPC messages statically.
>>> This is not such pain when RPC limits are small. However, if we want
>>> ever to increase those limits, we need to allocate buffer dynamically,
>>> based on RPC message len (= the first 4 bytes). Therefore we will
>>> decrease our mem usage in most cases and still be flexible enough in
>>> corner cases.
>>> ---
>>>  src/rpc/virnetclient.c       |   16 ++-
>>>  src/rpc/virnetmessage.c      |   12 ++-
>>>  src/rpc/virnetmessage.h      |    6 +-
>>>  src/rpc/virnetserverclient.c |   20 ++-
>>>  tests/virnetmessagetest.c    |  393 +++++++++++++++++++++++-------------------
>>>  5 files changed, 264 insertions(+), 183 deletions(-)
>>
>> I haven't looked closely at this, but I do have a generic question:
>>
>> Are you malloc'ing and free'ing the buffer for each rpc on every call,
>> or are you tracking a pool of buffers and only alloc'ing when needed?
>> Furthermore, can you use the stack or a statically allocated array to
>> receive short messages, saving alloc only for the long messages?  I'm
>> worried about performance if we malloc on every single RPC call.
> 
> To be honest I'm not convinced malloc() would be a big factor in the
> speed of the RPC calls. That said you could optimize it using the
> VIR_ALLOC_VAR  to allocate the virNetMessage struct + the buffer
> in one go. ALlocate all messages with 256kb initially and then you
> can relloc larger only if needed, and re-alloc back to 256 kb when
> done.
> 
> 
> Daniel

Okay, point taken. Although I'd rather avoid allocating virNetMessage
struct and buffer at once. It would complicate the code again, because
the client side code has this struct _virNetClient; which already
contains virNetMessage. However, if I'd need to re-alloc, I'd need to
realloc whole message (!) therefore change the member to
virNetMessagePtr. This implies too many changes.

Michal