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ram rw is only 50.87M bytes/sec??
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_______0
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PostPosted: Thu Feb 28, 2013 2:22 am    Post subject: ram rw is only 50.87M bytes/sec?? Reply with quote

hi,

I put an iso in /dev/shm and rsync there into another dir and transfer rate is only 50M/s. I think something wrong with this picture, SSD hard drives are faster. Shouldn't ram to ram copying be nearly instant for a gig file??

thnks
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Mad Merlin
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PostPosted: Thu Feb 28, 2013 5:41 am    Post subject: Reply with quote

Do you have enough free memory? If the system needs to swap memory to disk to make room for your file, you will obviously not be able to write faster than the system can swap (ie, very slow).

For reference, on my machine:

Code:

$ dd if=/dev/zero of=/dev/shm/bigfile bs=1M count=$[5*1024] conv=fdatasync
5120+0 records in
5120+0 records out
5368709120 bytes (5.4 GB) copied, 2.16982 s, 2.5 GB/s

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_______0
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PostPosted: Thu Feb 28, 2013 5:14 pm    Post subject: Reply with quote

Your command here:

5368709120 bytes (5.4 GB) copied, 2.65288 s, 2.0 GB/s

wait doing the file with dd

bytes (1.1 GB) copied, 0.719766 s, 1.5 GB/s

So, something is wrong with cp and rsync?? Why these two take longer?

Result with rsync:

1.10G 100% 51.24MB/s 0:00:20 (xfer#1, to-check=0/1)
sent 1.10G bytes received 31 bytes 53.46M bytes/sec

cp same behavior. How to make rsync transfer at the speeds of ram?

thanks

ps: what accounts with your extra .5 GB/s??? I have ddr3 here :/
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BitJam
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PostPosted: Fri Mar 01, 2013 3:57 am    Post subject: Re: ram rw is only 50.87M bytes/sec?? Reply with quote

_______0 wrote:
I put an iso in /dev/shm and rsync there into another dir and transfer rate is only 50M/s. I think something wrong with this picture, SSD hard drives are faster. Shouldn't ram to ram copying be nearly instant for a gig file??

Where is that other directory mounted? My guess is that it is mounted on a hard drive.
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Mad Merlin
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PostPosted: Wed Mar 06, 2013 1:38 am    Post subject: Reply with quote

_______0 wrote:
ps: what accounts with your extra .5 GB/s??? I have ddr3 here :/


There are a number of factors that would impact that speed, primarily, the CPU (type and speed), and the memory (type and number of channels). I have an i7 920 with triple channel DDR3.

Aside from Intel's high end desktop platform (socket 2011, quad channel memory), you pretty much only get dual channel memory nowadays, and since it's still DDR3, it's pretty much expected that it would have less memory bandwidth than my triple channel system, even though mine is over 4 years old now.
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