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Date: Fri, 3 Aug 2007 22:04:18 +0200 (CEST)
From: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>
To: Kris Kennaway <kris@obsecurity.org>
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Cc: freebsd-questions@freebsd.org
Subject: Re: parformance patch?
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>> -#define MAXPHYS                (128 * 1024)    /* max raw I/O transfer
>> size */
>> +#define MAXPHYS                (512 * 1024)    /* max raw I/O transfer
>> size */
>> both works for me and gives noticable speedup, when operating on big files
>> and when starting big apps or swapping.
>
> There are assumptions about the value of MAXPHYS all through the
> kernel.  I doubt this patch works properly - it is something that
> should be fixed more completely though.


funny but THIS patch (MAXPHYS, not other) is working for me on 4 heavily 
loaded servers for over a year with FreeBSD 6.2


while vm patch is running on my notebook for 2 days only, ok for now but 
still testing.

>
>> other question (swap_pager.c):
>>
>> /*
>>  * SWB_NPAGES must be a power of 2.  It may be set to 1, 2, 4, 8, or 16
>>  * pages per allocation.  We recommend you stick with the default of 8.
>>  * The 16-page limit is due to the radix code (kern/subr_blist.c).
>>  */
>>
>> #ifndef MAX_PAGEOUT_CLUSTER
>> #define MAX_PAGEOUT_CLUSTER 16
>> #endif
>>
>>
>>
>> can this be changed/fixed? 64-128 would be nicer...
>
> Why?  i.e. what are the implications of this, good and bad?
>

16*4=64kbytes

for modern disk drives 64 kbytes is transferred below 1 milisecond, while 
seek takes 8-10ms by average.

doing so small I/O is inefficient. something like 512kB (128 pages) looks 
better.


patched vm_fault make pageins faster, but not pageout. disks often does 
write caching clustering pageouts anyway, but probably not that efficient.