[systemd-devel] [RFC] [PATCH 0/3] journal: Add deferred log processing to reduce synchonous IO overhead

[Karol Lewandowski]

On 12/14/2013 04:47 AM, Lennart Poettering wrote:
> On Fri, 13.12.13 22:16, Karol Lewandowski (lmctlx at gmail.com) wrote:
>> On Fri, Dec 13, 2013 at 03:45:36PM +0100, Lennart Poettering wrote:
>>> On Fri, 13.12.13 12:46, Karol Lewandowski (k.lewandowsk at samsung.com) wrote:

>> One of the problems I see, though, is that no matter how deep I make
>> the queue (`max_dgram_qlen') I still see process sleeping on send()
>> way earlier that configured queue depth would suggest.

> It would be interesting to find out why this happens. I mean, there are
> three parameters here I could think of that matter: the qlen, SO_SNDBUF
> on the sender, and SO_RCVBUF on the receiver (though the latter two might
> actually change the same value on AF_UNIX? or maybe one of the latter
> two is a NOP on AF_UNIX?). If any of them reaches the limit then the
> sender will necessarily have to block.
> 
> (SO_SNDBUF and SO_RCVBUF can also be controlled via
> /proc/sys/net/core/rmem* and ../wmem*... For testing purposes it might
> be easier to play around with these and set them to ludicrously high
> values...)

That's it.

While journal code tries to set buffer size via SO_SNDBUF/SO_RCVBUF
options to 8MB, kernel limits these to wmem_max/rmem_max. On machines
I've tested respective values are quite small - around 150-200kB each.

Increasing these did reduce context switches considerably - preliminary
tests show that I can now queue thousands of messages (~5k) without
problems.  I will test this thoroughly in next few days.

I do wonder what is the rationale behind such low limits...

Thanks a lot!

Karol