netdev - Re: [RFC patch 1/1] large netlink dumps

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Date:   Sun, 16 Apr 2017 09:37:08 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Johannes Berg <johannes@...solutions.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [RFC patch 1/1] large netlink dumps

On Sun, 16 Apr 2017 09:03:08 -0400
Jamal Hadi Salim <jhs@...atatu.com> wrote:

> On 17-04-15 11:08 PM, Eric Dumazet wrote:
> > On Sat, 2017-04-15 at 13:07 -0400, Jamal Hadi Salim wrote:  
> >> Eric,
> >>
> >> How does attached look instead of the 32K?
> >> I found it helps to let user space suggest something
> >> larger.
> >>
> >> cheers,
> >> jamal  
> >
> > Looks dangerous to me, for various reasons.
> >
> > 1) Memory allocations might not like it
> >
> > Have you tried your change after user does a
> > setsockopt(    SO_RCVBUFFORCE,  256 Mbytes), and a
> > recvmsg ( .. 64 Mbytes) ?
> >
> > Presumably, we could replace 32768 by (PAGE_SIZE <<
> > PAGE_ALLOC_COSTLY_ORDER), but this will not matter on x86.
> >  
> 
> For my use case I dont need to go that high, but i can see
> plausibility that someone else will. Is there a reasonable
> large number other than 32K? 128K-512K would be way sufficient.

It was common with routing daemons to set SO_RCVBUF to very large values
to avoid losing notifications.

> > 2) We might have paths in the kernel filling a potential big skb without
> > yielding cpu or a spinlock or a mutex. -> latency source.
> >
> >
> > What perf numbers do you have, using 1MB buffers instead of 32KB ?
> >
> > The syscall overhead seems tiny compared to the actual cost of filling
> > the netlink message, accessing thousands of cache lines all over the
> > places.
> >  
> 
> sycall is affecting me - but I have only compared with limited
> traffic running at the same time as dumping. The more i can batch
> the sooner i can stop polluting the cache.
> 
> The tests I have done are with a default socket buffer of 4M
> and say recvmsg(... 128K). I dont need to go higher
> that 256-512K to achieve my goals.
> With default of 32K I can fit about 250-60 actions in one batch.
> With 128K I can fit 4x that.
> It takes about 1.5 minutes for one process to dump 1M actions
> on my laptop (Intel(R) Core(TM) i7-5500U CPU @ 2.40GHz) with
> 32K; 25% of that time with 128K. tc is single threaded, so i can
> keep one cpu busy 100% while I dump which means latency fear
> is lowered.
> 
> My eventual need: To dump all relevant stats every 5 seconds.
> I will send the other patch I talked about which filters based
> on time which helps in most cases but not always.
> 
> I am also now thinking of adding "a range index filter" and then
> multi-threading several parrallel requests, one for each range of
> indices.
> 
> cheers,
> jamal