Re: Kernel drops ethernet packets during disk writes

["Tiemen Schut" <tschut@gmail.com>]

Okay, I think I've found some sort of workaround/solution thingie. Not
really a solution though, it's more like a different approach of the
problem.

Instead of using pcap, I'm now using a socket interface on the
ethernet interface. Receiving packets this way and saving them to disk
(at a speed of 350Mbit/s), goes way better than using pcap!

With pcap, I lost like > 80% of the packets, using a raw socket I lose
like < 1% of the packets, which is a huge increase (well, actually a
decrease ;))!

So, to anybody trying to use pcap to capture packets and write 'm to
disk at the "same" time, I recommend using sockets instead of pcap.

TIemen


On 9/5/06, Tiemen Schut <tschut@gmail.com> wrote:
> Now this is something constructive!
>
> I can indeed maintain 350Mbit/s to disk (about 55MB/s to disk
> actually), and I can receive all the packets, as long as I don't try
> to do both things simultaniously.
>
> I'll try to increase the buffers tomorrow, will let you know how it
> worked out. I doubt though that the tcp buffers have any impact, we're
> talking raw ethernet frames here.
>
> Thanks,
>
> Tiemen
>
> On 9/5/06, Stephen Degler <stephen@degler.net > wrote:
> >  Tiemen Schut wrote:
> > > Summary: The linux kernel appears to drop raw ethernet packets if
> > > another process is writing to disk.
> > >
> > > Test environment: Used a p4 1.7 GHz with gigabit interface
> > > point-to-point connection to another p4 (windows pc). This windows PC
> > > generates raw ethernet frames holding a counter and sends 'm on to the
> > > linux PC, at a transfer rate of 350 Mbit/s. When not writing to disk,
> > > everything goes quite fine. I can check the counters at the linux
> > > side, and will notice a minimal packet loss (<  0.001 % or so).
> > >
> > > However, for my application I want to write each and every frame to
> > > disk. So, I created a second app, and through a fifo the receiver app
> > > and the disk writer app communicate. Now I'm losing like 80% of my
> > > packets :o Reducing the throughput on the network doesn't really help
> > > (though it does help a little).
> > >
> > > Note: I tried everything in the same app first, but my guess was that
> > > the write operation delayed the app, so I decided to put everything in
> > > two apps to give the scheduler some work ;) Didn't work though.
> > >
> > > My guess would be that the kernel drops the packets because the write
> > > operation takes to long (how long can it take, it's just a stupid 1512
> > > bytes frame). Anyway, I tried to enlarge
> > > .sys.net.core.netdev_max_backlog, but that didn't do the trick.
> > >
> > > This problem occures on both  2.6.13 and 2.4.idontremember.
> > >
> > > It kinda sucks, what's the use of receiving traffic if you can't write
> > > it to disk?
> > >
> > > I'm sending the packets using the winpcap library, and I'm receiving
> > > the packets using the pcap library.
> > >
> > > Any help would be _greatly_ appreciated, and if neccessary, please ask
> > > for additional information/used software/test results/etc.
> > >
> > > Tiemen Schut
> > Hi your packets are being dropped perhaps because there is no room to
> > store them while the disk is busy, not that you don't receive them.
> >
> > Can you sustain 35M/second to disk?  If you can't do that with dd
> >  if=/dev/zero of="foo" bs=1024k, then you can not expect to do it with
> > the network plus your writer.  Writing
> > sequentially to a large modern disk you should be able to do this (large
> > capacity SATA or SCSI disks, or hardware raid controllers).
> >
> > You can improve matters greatly by increasing the kernel network buffers
> > available.  This will give you more buffer room while the disk subsystem
> > is busy.  Look at the following tuneables: you should be able to greatly
> > reduce packet loss by increasing them from the default values.
> >
> > net.core.rmem_max = 2500000
> > net.core.wmem_max = 2500000
> > net.ipv4.tcp_rmem = 4096 5000000 5000000
> > net.ipv4.tcp_wmem = 4096 65536 5000000
> >
> > I can assure you that with the proper system configuration, high network
> > to disk throughput is indeed possible.
> >
> > Hope this helps,
> > skd
> >
> >
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