smbd: High CPU usage

[Volker Lendecke]

On Wed, Jul 18, 2018 at 04:30:06PM +0300, Vadim Lazovskiy via samba-technical wrote:
> I've encountered the issue in samba4. I'm totally convinced it is a bug.
> In *some* cases with *some* clients smbd process hogs the CPU.
> Attaching strace to such process shows huge amount sendfile() syscall
> that's trying to write to non-blocking socket while write is actually would
> block:
> 
> sendfile(37, 34, [320625896], 238360)   = -1 EAGAIN (Resource temporarily
> unavailable)
> sendfile(37, 34, [320625896], 238360)   = -1 EAGAIN (Resource temporarily
> unavailable)
> sendfile(37, 34, [320625896], 238360)   = -1 EAGAIN (Resource temporarily
> unavailable)
> sendfile(37, 34, [320625896], 238360)   = -1 EAGAIN (Resource temporarily
> unavailable)
> sendfile(37, 34, [320625896], 238360)   = -1 EAGAIN (Resource temporarily
> unavailable)
> 
> Among this flood there are some rare successful writes:
> sendfile(37, 34, [320497416] => [320561656], 366840) = 64240
> 
> 37 is the network descriptor
> 34 is the file descriptor.
> 
> I've faced this issue several years ago when upgraded samba to 4.0. Then
> I've decided to downgrade to 3.5. But now there is no way to stay at 3.x.
> 
> Of course it is possible to completely disable sendfile using "use sendfile
> = no", then samba starts to use read and write syscalls but it would show
> some performance decline.

Unless you are on an embedded platform like ARM or MIPS you should not
notice any performance decline. I would suggest disabling sendfile.

Volker

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