Re: HIGHMEM4G config for 1GB RAM on desktop?

[Timothy Miller <miller@techsource.com>]

Timothy Miller wrote:
> 
> 
> Andrew Morton wrote:
> 
>>
>>
>> The 896M/128M split has a bit of a problem now each zone has its own LRU:
>> the size of the highmem zone is less than the amount of memory which is
>> described by the default /proc/sys/vm/dirty_ratio.  So it is easy to
>> completely fill highmem with dirty pages.  This causes a fairly large
>> amount of writeback via vmscan.c's writepage().  This causes poor I/O
>> submission patterns.  This causes a simple large, linear `dd' write to 
>> run
>> at only 50-70% of disk bandwidth.  (This was 6-12 months ago - it 
>> might be
>> a bit better now)
>>
> 
> 
> Hey, that rings a bell.  I have a 3ware 7000-2 controller with two 
> WD1200JB drives in RAID1.  I find that if I dd from the disk, I get 
> exactly the read throughput that is the max for the drives (47MB/sec). 
> However, if I do a WRITE test, the performance is miserable.
> 
> I have been going back and forth with 3ware for months, and what's odd 
> is that my drives with my controller in any machine other than the 
> primary box get great write throughput, BUT on my main box with 1G of 
> RAM, I get MISERABLE write throughput.  When I should be getting 
> 36MB/sec or faster, I get 8 to 12 MB/sec.
> 
> Now, I have tried limiting the memory with a mem= boot option, but that 
> doesn't change the performance any.
>

Scratch all this.  Even if I physically remove half the memory, I STILL 
get the performance problem.