On Mon, 1 Nov 2010, Takuya Yoshikawa wrote: > (2010/10/31 3:28), Jesper Juhl wrote: > > Hi, > > > > We can improve kvm_vm_ioctl_get_dirty_log() slightly by using vzalloc() > > rather than first allocating and then manually zero the memory with > > memset(). Also, while I was looking at this I noticed that we assign > > I personally prefer this new vzalloc() to vmalloc() + memset(). > > Just from my interest, is there real performance difference not just > the cleanup effect? If so, we'd better do this for other places too. > There's definately a positive size impact for the generated object code and we save having to do the call to memset() and the cost of a vzalloc() call looks more or less the same as a call to vmalloc() to me. <snip> > > This patch is not based on kvm.git, I guess. > Nope. It was generated against mainline git as of a couple of days ago. I can generate a version against kvm.git if you prefer. > > r = -ENOMEM; > > - dirty_bitmap = vmalloc(n); > > + dirty_bitmap = vzalloc(n); > > if (!dirty_bitmap) > > goto out; > > - memset(dirty_bitmap, 0, n); > > > > - r = -ENOMEM; > > This one is here because this belongs to a different code block from the > previous one. This keeps it easy to insert another codes in between these > two blocks. The optimization will be done at compile time. > IIRC, I did like this based on Avi's advise. > Fair enough, I've dropped that bit from the patch. Updated version below (against current mainline git). Signed-off-by: Jesper Juhl <jj@xxxxxxxxxxxxx> --- x86.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 2288ad8..624d4da 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3174,10 +3174,9 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, spin_unlock(&kvm->mmu_lock); r = -ENOMEM; - dirty_bitmap = vmalloc(n); + dirty_bitmap = vzalloc(n); if (!dirty_bitmap) goto out; - memset(dirty_bitmap, 0, n); r = -ENOMEM; slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL); -- Jesper Juhl <jj@xxxxxxxxxxxxx> http://www.chaosbits.net/ Plain text mails only, please http://www.expita.com/nomime.html Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
