svn commit: r270759 - in head/sys: cddl/compat/opensolaris/kern cddl/compat/opensolaris/sys cddl/contrib/opensolaris/uts/common/fs/zfs vm

[Steven Hartland]

----- Original Message ----- 
From: "Peter Wemm" <peter at wemm.org>
> On Friday 29 August 2014 20:51:03 Steven Hartland wrote:
snip..

> > Does Karl's explaination as to why this doesn't work above change 
> > your
> > mind?
>
> Actually no, I would expect the code as committed would *cause* the
> undesirable behavior that Karl described.
>
> ie: access a few large files and cause them to reside in cache.  Say 
> 50GB or so
> on a 200G ram machine.  We now have the state where:
>
> v_cache = 50GB
> v_free = 1MB
>
> The rest of the vm system looks at vm_paging_needed(), which is:  do 
> we have
> enough "v_cache + v_free"?  Since there's 50.001GB free, the answer is 
> no.
> It'll let v_free run right down to v_free_min because of the giant 
> pool of
> v_cache just sitting there, waiting to be used.
>
> The zfs change, as committed will ignore all the free memory in the 
> form of
> v_cache.. and will be freaking out about how low v_free is getting and 
> will be
> sacrificing ARC in order to put more memory into the v_free pool.
>
> As long as ARC keeps sacrificing itself this way, the free pages in 
> the v_cache
> pool won't get used.  When ARC finally runs out of pages to give up to 
> v_free,
> the kernel will start using the free pages from v_cache.  Eventually 
> it'll run
> down that v_cache free pool and arc will be in a bare minimum state 
> while this
> is happening.
>
> Meanwhile, ZFS ARC will be crippled.  This has consequences - it does 
> RCU like
> things from ARC to keep fragmentation under control.  With ARC 
> crippled,
> fragmentation will increase because there's less opportunistic 
> gathering of
> data from ARC.
>
> Granted, you have to get things freed from active/inactive to the 
> cache state,
> but once it's there, depending on the worlkload, it'll mess with ARC.

There's already a vm_paging_needed() check in there below so this will 
already
be dealt with will it not?

    Regards
    Steve