Re: efficient copy_to_user and copy_from_user routines in Linux Kernel

[Mala Anand]

>Mala Anand wrote:
>>
>> Here is a 2.5.19 patch that improves the performance of IA32
copy_to_user
>> and copy_from_user routines used by :
>>
>> (1) tcpip protocol stack
>> (2) file systems
>>

>This came up about a year back when zerocopy networking was merged.
>Intel boxes started running more slowly purely because of the 8+8
>alignment thing.

>I changed tcp to use a different copy if either source or dest were
>not eight-byte aligned, and found that the resulting improvement
>across a mixed networking load was only 1%.  Your numbers are higher,
>so perhaps there are different alignments in the mix...

I will test on other workloads when I return back to work after OLS
and vacation.  However we tested an earlier version of this patch on
Netbench using sendfile and gained around 3% improvement. The baseline
profiling showed that Netbench was spending 10% in generic_copy_to_user.
The tcp options are aligned on an 4-byte boundary, so depending on the
options used the address to the data (source address to the
generic_copy_to_user) should fall on an 4 or 8 byte boundary. I agree
with you more test is needed.

>One question:  have you tested on other CPU types?  This problem is
>very specific to Intel hardware.  On AMD, the eight-byte alignement
>artifact does not exist at all.  It could be that your patch is not
>desirable on such CPUs?

I tested only Pentium II and III. I will test it on Pentium IV.
When I said 8-byte alignment, it is 8 and greater.  I will
try to check out AMD also.

Regards,
    Mala

   Mala Anand
   E-mail:[e-mail]
   Linux Technology Center - Performance
   Phone:[phone]; Tie-line:678-8088

                                                                                                                                       
                      Andrew Morton                                                                                                    
                      <[e-mail]>        To:       Mala Anand/Austin/IBM@IBMUS                                                   
                      Sent by:                 cc:       Linux Kernel Mailing List <[e-mail]>,                     
                      [e-mail]           [e-mail]                                                         
                                               Subject:  Re: efficient copy_to_user and copy_from_user routines in Linux Kernel        
                                                                                                                                       
                      06/25/2002 12:03                                                                                                 
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