Messages in this thread | | | Date | Sun, 28 Jan 2001 16:34:51 +1100 | From | Andrew Morton <> | Subject | Re: sendfile+zerocopy: fairly sexy (nothing to do with ECN) |
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kuznet@ms2.inr.ac.ru wrote: > > Hello! > > > 2.4.1-pre10+zercopy, using read()/write(): 38.1% CPU > > write() on zc card is worse than normal write() by definition. > It generates split buffers.
yes. The figures below show this. Disabling SG+checksums speeds up write() and send().
> Split buffers are more expensive and we have to pay for this. > You have paid too much for slow card though. 8) > > Do you measure load correctly?
Yes. Quite confident about this. Here's the algorithm:
1: Run a cycle-soaker on each CPU on an otherwise unloaded system. See how much "work" they all do per second.
2: Run the cycle-soakers again, but with network traffic happening. See how much their "work" is reduced. Deduce networking CPU load from this difference.
The networking code all runs SCHED_FIFO or in interrupt context, so the cycle-soakers have no effect upon the network code's access to the CPU.
The "cycle-soakers" just sit there spinning and dirtying 10,000 cachelines per second.
> > 2.4.1-pre10+zercopy, using read()/write(): 39.2% CPU * hardware tx checksums disabled > > This is illegal combination of parameters. You force two memory accesses, > doing this. The fact that it does not add to load is dubious. 8)8)
mm.. Perhaps with read()/write() the data is already in cache?
Anyway, I've tweaked up the tool again so it can do send() or write() (then I looked at the implementation and wondered why I'd bothered). It also does TCP_CORK now.
I ran another set of tests. The zerocopy patch improves sendfile() hugely but slows down send()/write() significantly, with a 3c905C:
http://www.uow.edu.au/~andrewm/linux/#zc
The kernels which were tested were 2.4.1-pre10 with and without the zerocopy patch. We only look at client load (the TCP sender).
In all tests the link throughput was 11.5 mbytes/sec at all times (saturated 100baseT) unless otherwise noted.
The client (the thing which sends data) is a dual 500MHz PII with a 3c905C.
For the write() and send() tests, the chunk size was 64 kbytes.
The workload was 63 files with an average length of 350 kbytes.
CPU
2.4.1-pre10+zerocopy, using sendfile(): 9.6% 2.4.1-pre10+zerocopy, using send(): 24.1% 2.4.1-pre10+zerocopy, using write(): 24.2%
2.4.1-pre10+zerocopy, using sendfile(): 16.2% * checksums and SG disabled 2.4.1-pre10+zerocopy, using send(): 21.5% * checksums and SG disabled 2.4.1-pre10+zerocopy, using write(): 21.5% * checksums and SG disabled
2.4.1-pre10-vanilla, using sendfile(): 17.1% 2.4.1-pre10-vanilla, using send(): 21.1% 2.4.1-pre10-vanilla, using write(): 21.1%
Bearing in mind that a large amount of the load is in the device driver, the zerocopy patch makes a large improvement in sendfile efficiency. But read() and send() performance is decreased by 10% - more than this if you factor out the constant device driver overhead.
TCP_CORK makes no difference. The files being sent are much larger than a single frame.
Conclusions:
For a NIC which cannot do scatter/gather/checksums, the zerocopy patch makes no change in throughput in all case.
For a NIC which can do scatter/gather/checksums, sendfile() efficiency is improved by 40% and send() efficiency is decreased by 10%. The increase and decrease caused by the zerocopy patch will in fact be significantly larger than these two figures, because the measurements here include a constant base load caused by the device driver.
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