linux-kernel - Re: [benchmark] 1% performance overhead of paravirt

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Message-ID: <4A2E5BF1.5060405@redhat.com>
Date:	Tue, 09 Jun 2009 15:56:17 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Nick Piggin <npiggin@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native
 kernels

Ingo Molnar wrote:
> Fork is pretty much the worst-case measurement for larger pte 
> overhead, as it has to copy around a lot of pagetables.
>   

We could eliminate that if we use the R/W bit on pgd entries.  fork() 
would be 256 clear_bit()s (1536 and 768 on i386 pae and nonpae).

copy_one_pte() disagrees though:

        if (unlikely(!pte_present(pte))) {
                if (!pte_file(pte)) {
                        swp_entry_t entry = pte_to_swp_entry(pte);

                        swap_duplicate(entry);
                        /* make sure dst_mm is on swapoff's mmlist. */
                        if (unlikely(list_empty(&dst_mm->mmlist))) {
                                spin_lock(&mmlist_lock);
                                if (list_empty(&dst_mm->mmlist))
                                        list_add(&dst_mm->mmlist,
                                                 &src_mm->mmlist);
                                spin_unlock(&mmlist_lock);
                        }
                        if (is_write_migration_entry(entry) &&
                                        is_cow_mapping(vm_flags)) {
                                /*
                                 * COW mappings require pages in both parent
                                 * and child to be set to read.
                                 */
                                make_migration_entry_read(&entry);
                                pte = swp_entry_to_pte(entry);
                                set_pte_at(src_mm, addr, src_pte, pte);
                        }
                }
                goto out_set_pte;
        }


Not sure how we can enlaze this thing.

-- 
error compiling committee.c: too many arguments to function

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