amd64: change VM_KMEM_SIZE_SCALE to 1?

[John Baldwin]

On Saturday, July 31, 2010 5:39:39 pm Alan Cox wrote:
> John Baldwin wrote:
> > On Friday, July 30, 2010 2:49:59 pm Alan Cox wrote:
> >> With that in mind, the following patch slows the growth of "virt" from 
> >> 2/5 of vm_kmem_size to 1/7.  This has no effect on amd64.  However, on 
> >> i386. it allows desiredvnodes to grow slowly for machines with 1.5GB to 
> >> about 2.5GB of RAM, ultimately exceeding the old desiredvnodes cap by 
> >> about 17%.  Once we exceed the old cap, we increase desiredvnodes at a 
> >> marginal rate that is almost the same as your patch, about 1% of 
> >> physical memory.  It's just computed differently.
> >>
> >> Using 1/8 instead of 1/7, amd64 machines with less than about 1.5GB lose 
> >> about 7% of their vnodes, but they catch up and pass the old limit by 
> >> 1.625GB.  Perhaps, more importantly, i386 machines only exceed the old 
> >> cap by 3%.
> >>
> >> Thoughts?
> >
> > I think this is much better.  My strawman was rather hackish in that it was
> > layering a hack on top of the existing calculations.  I prefer your approach.
> > I do not think penalizing amd64 machines with less than 1.5GB is a big worry
> > as most x86 machines with a small amount of memory are probably running as
> > i386 anyway.  Given that, I would probably lean towards 1/8 instead of 1/7,
> > but I would be happy with either one.
> 
> I've looked a bit at an i386/PAE system with 8GB.  I don't think that a 
> default configuration, e.g., no changes to the mbuf limits, is at risk 
> with 1/7.

Ok.

> > How is this value computed?  I would prefer something like:
> >
> > '512 * 1024 * 1024 * 1024 / (sizeof(struct vnode) + sizeof(struct vm_object) / N'
> >
> > if that is how it is computed.  A brief note about the magic number of 393216
> > would also be nice to have (and if it could be a constant with a similar
> > formula value that would be nice, too.).
> >
> >   
> 
> I've tried to explain this computation below.

Thanks, it looks good to me now.

> Index: kern/vfs_subr.c
> ===================================================================
> --- kern/vfs_subr.c     (revision 210702)
> +++ kern/vfs_subr.c     (working copy)
> @@ -282,23 +282,34 @@ SYSCTL_INT(_debug, OID_AUTO, vnlru_nowhere, CTLFLA
>  
>  /*
>   * Initialize the vnode management data structures.
> + *
> + * Reevaluate the following cap on the number of vnodes after the physical
> + * memory size exceeds 512GB.  In the limit, as the physical memory size
> + * grows, the ratio of physical pages to vnodes approaches sixteen to one.
>   */
>  #ifndef        MAXVNODES_MAX
> -#define        MAXVNODES_MAX   100000
> +#define        MAXVNODES_MAX   (512 * (1024 * 1024 * 1024 / PAGE_SIZE / 
> 16))
>  #endif
>  static void
>  vntblinit(void *dummy __unused)
>  {
> +       int physvnodes, virtvnodes;
>  
>         /*
> -        * Desiredvnodes is a function of the physical memory size and
> -        * the kernel's heap size.  Specifically, desiredvnodes scales
> -        * in proportion to the physical memory size until two fifths
> -        * of the kernel's heap size is consumed by vnodes and vm
> -        * objects.
> +        * Desiredvnodes is a function of the physical memory size and the
> +        * kernel's heap size.  Generally speaking, it scales with the
> +        * physical memory size.  The ratio of desiredvnodes to physical 
> pages
> +        * is one to four until desiredvnodes exceeds 98,304.  
> Thereafter, the
> +        * marginal ratio of desiredvnodes to physical pages is one to
> +        * sixteen.  However, desiredvnodes is limited by the kernel's heap
> +        * size.  The memory required by desiredvnodes vnodes and vm objects
> +        * may not exceed one seventh of the kernel's heap size.
>          */
> -       desiredvnodes = min(maxproc + cnt.v_page_count / 4, 2 * 
> vm_kmem_size /
> -           (5 * (sizeof(struct vm_object) + sizeof(struct vnode))));
> +       physvnodes = maxproc + cnt.v_page_count / 16 + 3 * min(98304 * 4,
> +           cnt.v_page_count) / 16;
> +       virtvnodes = vm_kmem_size / (7 * (sizeof(struct vm_object) +
> +           sizeof(struct vnode)));
> +       desiredvnodes = min(physvnodes, virtvnodes);
>         if (desiredvnodes > MAXVNODES_MAX) {
>                 if (bootverbose)
>                         printf("Reducing kern.maxvnodes %d -> %d\n",
> 
> 

-- 
John Baldwin