Lustre discuss - Mar 2008 - How to you generated discarded readahead pages?

I''m running a little script that does 3 consecutive small reads of 4K 
each and then sleeps for a second and as you can see from the following 
collectl output readahead is happening.  How do I know?  Because the 
network traffic is so high:

#         <----------Network----------><-------------Lustre 
Client-------------->
#Time     netKBi pkt-in netKBo pkt-out  Reads KBRead Writes KBWrite   
Hits Misses
09:31:54    1084    740     29     394      3     12      0       0      
0      3
09:31:55    1084    740     29     394      3     12      0       0      
0      3
09:31:56    1084    740     29     394      3     12      0       0      
0      3
09:31:57    1084    741     29     394      2      8      0       0      
0      2

I''m getting my data from 
/proc/fs/lustre/llite/spfs1-00000100768c2800/read_ahead_stats and when I 
look at the disards, they''re not changing.  What I would like to know
is
why. as I''d think all that extra data would be discarded from the 
cache.  Is it because some other process needs to put pressure on it?  I 
have 4GB memory and just wrote a 5GB file which should certain force out 
anything already there but the counter never changed.

I do see "readpage not consecutive" changing,

-mark

Mark Seger wrote:
> I''m running a little script that does 3 consecutive small reads of
4K
> each and then sleeps for a second and as you can see from the following 
> collectl output readahead is happening.  How do I know?  Because the 
> network traffic is so high:
>
> #         <----------Network----------><-------------Lustre 
> Client-------------->
> #Time     netKBi pkt-in netKBo pkt-out  Reads KBRead Writes KBWrite   
> Hits Misses
> 09:31:54    1084    740     29     394      3     12      0       0      
> 0      3
> 09:31:55    1084    740     29     394      3     12      0       0      
> 0      3
> 09:31:56    1084    740     29     394      3     12      0       0      
> 0      3
> 09:31:57    1084    741     29     394      2      8      0       0      
> 0      2
>
> I''m getting my data from 
> /proc/fs/lustre/llite/spfs1-00000100768c2800/read_ahead_stats and when I 
> look at the disards, they''re not changing.  What I would like to
know is
> why. as I''d think all that extra data would be discarded from the 
> cache.  Is it because some other process needs to put pressure on it? 
These read-ahead pages will be discarded when it met memory pressure or
the client revokes the lock of these pages.
>  I 
> have 4GB memory and just wrote a 5GB file which should certain force out 
> anything already there but the counter never changed.
>   
If you did not change the parameters in /proc, lustre client only permit 
32M dirty cache currently,
you can check max_dirty_mb in /proc . So writing 5GB file will not 
trigger memory pressure on client.
> I do see "readpage not consecutive" changing,
>
> -mark
>
>
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