[Libwebsockets] lws_write speed

Andy Green andy at warmcat.com
Wed May 18 13:38:06 CEST 2016

On 05/18/2016 07:08 PM, Denis Osvald wrote:
> Hi all,
> On 05/16/2016 09:33 PM, Pierre Laplante wrote:
>> In the example of the server, you are using malloc to copy the data
>> before using lws_write as in:
>> ...
>> Is there another way to do this ?
> Andy, have you looked at the writev syscall, if it's usable in this
> scenario?
> It accepts multiple buffers, so it could maybe use the first buffer for
> the padding thus not requiring user to have padding space in their
> buffer. It would send them both at once in one writev call then.
> Maybe it's worth exploring?

I think it doesn't help with the basic problem, which is that the kernel 
must restrict how much memory it sets aside for buffering a specific 
networking connection.

Actually writev() semantics seem to be no different than write() in the 
sense it will inform you afterwards how much it actually used

        On success, readv() and preadv() return the number of bytes 
read; writev() and pwritev() return the number of bytes writ‐
        ten.  On error, -1 is returned, and errno is set appropriately.

The basic problem is that the kernel may conclude it only wants to take 
a little of your data into kernelside buffers based on global memory 
situation and the conditions on the specific connection.

writev() won't magically solve that if I understood it, it's more about 
matching kernelside scatter-gather semantics when there is no special 
restriction on buffering from the driver (ie, storage).


> Regards,
> Denis
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