Re: [PATCH 18/31] powerpc/mm: Increase the pte frag size.
From: Aneesh Kumar K.V <hidden>
Date: 2015-09-21 08:45:23
Benjamin Herrenschmidt [off-list ref] writes:
On Mon, 2015-09-21 at 12:10 +0530, Aneesh Kumar K.V wrote:quoted
/* - * We use a 2K PTE page fragment and another 2K for storing - * real_pte_t hash index + * We use a 2K PTE page fragment and another 4K for storing + * real_pte_t hash index. Rounding the entire thing to 8K */Isn't this a LOT of memory wasted ? Page tables have a non-negligible footprint, we were already wasting half, now we are wasting 3/4 no ?
The actual math is, we used to allocate 16 PTE page from a 64K page before. We now do 8 pte page from a 64K linux page.
Ie, in most cases on modern machines we never use the other "half"...
That is true. We will use this only when we use 4K subpage. But I am not sure there is a better solution. Also, we should find this slightly imporve our contention on ptl lock. With SPLIT_PTLOCK we now have less number of pte page using the same spin lock. -aneesh