Messages in this thread Patch in this message |  | | Subject | [PATCH] low-latency zap_page_range | From | Robert Love <> | Date | 20 Jul 2002 13:20:26 -0700 |
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The lock hold time in zap_page_range is horrid. This patch breaks the work up into chunks and relinquishes the lock after each iteration. This drastically lowers latency by creating a preemption point, as well as lowering lock contention.
The chunk size is ZAP_BLOCK_SIZE and currently 256*PAGE_SIZE.
This lowers the maximum latency in zap_page_range from 10~20ms (on a dual Athlon - one of the worst latencies recorded) to unmeasurable.
I made a couple other cleanups and optimizations:
- remove unneeded dir variable and call to pgd_offset - nothing uses this anymore as the code was pushed to unmap_page_range
- removed duplicated start variable - it is the same as address
- BUG -> BUG_ON in unmap_page_range
- remove redundant BUG from zap_page_range - the same check is done in unmap_page_range
- better comments
Patch is against 2.5.27, please apply.
Robert Love
diff -urN linux-2.5.27/mm/memory.c linux/mm/memory.c --- linux-2.5.27/mm/memory.c Sat Jul 20 12:11:17 2002 +++ linux/mm/memory.c Sat Jul 20 12:56:18 2002 @@ -390,8 +390,8 @@ { pgd_t * dir; - if (address >= end) - BUG(); + BUG_ON(address >= end); + dir = pgd_offset(vma->vm_mm, address); tlb_start_vma(tlb, vma); do { @@ -402,34 +402,42 @@ tlb_end_vma(tlb, vma); } -/* - * remove user pages in a given range. +#define ZAP_BLOCK_SIZE (256 * PAGE_SIZE) /* how big a chunk we loop over */ + +/** + * zap_page_range - remove user pages in a given range + * @vma: vm_area_struct holding the applicable pages + * @address: starting address of pages to zap + * @size: number of bytes to zap */ void zap_page_range(struct vm_area_struct *vma, unsigned long address, unsigned long size) { struct mm_struct *mm = vma->vm_mm; mmu_gather_t *tlb; - pgd_t * dir; - unsigned long start = address, end = address + size; - - dir = pgd_offset(mm, address); + unsigned long end, block; /* - * This is a long-lived spinlock. That's fine. - * There's no contention, because the page table - * lock only protects against kswapd anyway, and - * even if kswapd happened to be looking at this - * process we _want_ it to get stuck. + * This was once a long-held spinlock. Now we break the + * work up into ZAP_BLOCK_SIZE units and relinquish the + * lock after each interation. This drastically lowers + * lock contention and allows for a preemption point. */ - if (address >= end) - BUG(); - spin_lock(&mm->page_table_lock); - flush_cache_range(vma, address, end); - - tlb = tlb_gather_mmu(mm, 0); - unmap_page_range(tlb, vma, address, end); - tlb_finish_mmu(tlb, start, end); - spin_unlock(&mm->page_table_lock); + while (size) { + block = (size > ZAP_BLOCK_SIZE) ? ZAP_BLOCK_SIZE : size; + end = address + block; + + spin_lock(&mm->page_table_lock); + + flush_cache_range(vma, address, end); + tlb = tlb_gather_mmu(mm, 0); + unmap_page_range(tlb, vma, address, end); + tlb_finish_mmu(tlb, address, end); + + spin_unlock(&mm->page_table_lock); + + address += block; + size -= block; + } } /* - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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