Re: [PATCH userspace v2] libsepol: cache ebitmap cardinality value

From: Nicolas Iooss <nicolas.iooss@m4x.org>
To: Ondrej Mosnacek <omosnace@redhat.com>,
	SElinux list <selinux@vger.kernel.org>,
	Stephen Smalley <sds@tycho.nsa.gov>
Subject: Re: [PATCH userspace v2] libsepol: cache ebitmap cardinality value
Date: Tue, 25 Feb 2020 22:24:58 +0100	[thread overview]
Message-ID: <CAJfZ7==_-vg1e3LV-Wbf94PjFu86pxc3-J6BqJ_Qa73uaj+pkw@mail.gmail.com> (raw)
In-Reply-To: <CAFqZXNuvL1KQwahXPJKngr_p7sON0SNM=5Mecao0QhAGfRTN2g@mail.gmail.com>

On Tue, Feb 18, 2020 at 5:01 PM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>
> On Tue, Feb 18, 2020 at 4:40 PM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > On 2/18/20 10:22 AM, Ondrej Mosnacek wrote:
> > > On Thu, Feb 13, 2020 at 2:40 PM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > >> According to profiling of semodule -BN, ebitmap_cardinality() is called
> > >> quite often and contributes a lot to the total runtime. Cache its result
> > >> in the ebitmap struct to reduce this overhead. The cached value is
> > >> invalidated on most modifying operations, but ebitmap_cardinality() is
> > >> usually called once the ebitmap doesn't change any more.
> > >>
> > >> After this patch, the time to do 'semodule -BN' on Fedora Rawhide has
> > >> decreased from ~14.6s to ~12.4s (2.2s saved).
> > >
> > > I have no idea why, but I'm now getting completely different times
> > > (10.9s vs. 8.9s) with the same builds on the same setup... I can no
> > > longer reproduce the slower times anywhere (F31/locally/...) so I have
> > > to assume it was some kind of glitch. Since the numbers show a similar
> > > magnitude of speed-up (and they depend on a bunch of HW/SW factors
> > > anyway), I'm not going to do another respin. The applying person (most
> > > likely Stephen) is free to fix the numbers when applying if they wish
> > > to do so.
> >
> > Thanks, applied with fixed times (although I don't really think it
> > matters very much).  Maybe you're also picking up the difference from
> > the "libsepol/cil: remove unnecessary hash tables" change.
>
> No, that was actually the reason for the first correction.

Hello,
About performance issues, the current implementation of
ebitmap_cardinality() is quadratic:

for (i=ebitmap_startbit(e1); i < ebitmap_length(e1); i++)
    if (ebitmap_get_bit(e1, i))
        count++;

... because ebitmap_get_bit() browse the bitmap:

while (n && (n->startbit <= bit)) {
   if ((n->startbit + MAPSIZE) > bit) {
      /*... */

A few years ago, I tried modifying this function to make it linear in
the bitmap size:

unsigned int ebitmap_cardinality(ebitmap_t *e1)
{
    unsigned int count = 0;
    ebitmap_node_t *n;

   for (n = e1->node; n; n = n->next) {
        count += __builtin_popcountll(n->map);
    }
    return count;
}

... but never actually sent a patch for this, because I wanted to
assess how __builtin_popcountll() was supported by several compilers
beforehand. Would this be helpful to gain even more performance gain?

Cheers,
Nicolas