Re: [Qemu-devel] [PATCH] PCI: minor performance optimization

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 20, 2015 at 07:58:01PM +0800, Cao jin wrote:
> 
> 
> On 11/20/2015 07:26 PM, Michael S. Tsirkin wrote:
> >On Fri, Nov 20, 2015 at 07:04:07PM +0800, Cao jin wrote:
> >>
> >>
> >>On 11/20/2015 06:45 PM, Michael S. Tsirkin wrote:
> >>>On Fri, Nov 20, 2015 at 06:45:01PM +0800, Cao jin wrote:
> >>>
> >>>>2. As spec says, each capability must be DWORD aligned, so an optimization can
> >>>>    be done via Loop Unrolling.
> >>>
> >>>Why do we want to optimize it?
> >>>
> >>
> >>For tiny performance improvement via less loop. take pcie express
> >>capability(60 bytes at most) for example, it may loop 60 times, now we just
> >>need 15 times, a quarter of before.
> >
> >But who cares? This is not a data path operation.
> 
> It is tiny thing I found when browsing code. When found there are several
> places looks like this, I think maybe it does good to qemu to do this and
> CCed to you because it don`t look like a simple trivial patch.
> 
> So, hey Michael, if you don`t like this kind of optimization, that`t ok,
> forget it. But I think it make me little confused when determine which kind
> of patch should be CCed to you.

Optimization patches should normally include performance numbers
if they are to be merged.
Try to come up with a benchmark and you will realize that the speed of
this function has no effect under even half way realistic conditions.

> >
> >>>>
> >>>>Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
> >>>>---
> >>>>  hw/pci/pci.c | 12 ++++++++----
> >>>>  1 file changed, 8 insertions(+), 4 deletions(-)
> >>>>
> >>>>diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> >>>>index 168b9cc..1e99603 100644
> >>>>--- a/hw/pci/pci.c
> >>>>+++ b/hw/pci/pci.c
> >>>>@@ -1924,13 +1924,15 @@ PCIDevice *pci_create_simple(PCIBus *bus, int devfn, const char *name)
> >>>>  static uint8_t pci_find_space(PCIDevice *pdev, uint8_t size)
> >>>>  {
> >>>>      int offset = PCI_CONFIG_HEADER_SIZE;
> >>>>-    int i;
> >>>>-    for (i = PCI_CONFIG_HEADER_SIZE; i < PCI_CONFIG_SPACE_SIZE; ++i) {
> >>>>+    int i = PCI_CONFIG_HEADER_SIZE;;
> >>>>+
> >>>>+    for (; i < PCI_CONFIG_SPACE_SIZE; i = i + 4) {
> >>>>          if (pdev->used[i])
> >>>>-            offset = i + 1;
> >>>>-        else if (i - offset + 1 == size)
> >>>>+            offset = i + 4;
> >>>>+        else if (i - offset >= size)
> >>>>              return offset;
> >>>>      }
> >>>>+
> >>>>      return 0;
> >>>>  }
> >>>>
> >>>>@@ -2144,6 +2146,8 @@ int pci_add_capability2(PCIDevice *pdev, uint8_t cap_id,
> >>>>      uint8_t *config;
> >>>>      int i, overlapping_cap;
> >>>>
> >>>>+    assert(size > 0);
> >>>>+
> >>>>      if (!offset) {
> >>>>          offset = pci_find_space(pdev, size);
> >>>>          if (!offset) {
> >>>>--
> >>>>2.1.0
> >>>.
> >>>
> >>
> >>--
> >>Yours Sincerely,
> >>
> >>Cao Jin
> >.
> >
> 
> -- 
> Yours Sincerely,
> 
> Cao Jin