[Lustre-discuss] How to you generated discarded readahead pages?

[Tom.Wang]

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
>
>
> _______________________________________________
> Lustre-discuss mailing list
> Lustre-discuss at lists.lustre.org
> http://lists.lustre.org/mailman/listinfo/lustre-discuss
>