linux-kernel - Re: [PATCH v19 3/7] xbitmap: add more operations

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Date:   Sat, 16 Dec 2017 13:31:24 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mst@...hat.com
Cc:     willy@...radead.org, wei.w.wang@...el.com,
        virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mhocko@...nel.org,
        akpm@...ux-foundation.org, mawilcox@...rosoft.com,
        david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com, nilal@...hat.com, riel@...hat.com
Subject: Re: [PATCH v19 3/7] xbitmap: add more operations

Michael S. Tsirkin wrote:
> On Sat, Dec 16, 2017 at 01:21:52AM +0900, Tetsuo Handa wrote:
> > My understanding is that virtio-balloon wants to handle sparsely spreaded
> > unsigned long values (which is PATCH 4/7) and wants to find all chunks of
> > consecutive "1" bits efficiently. Therefore, I guess that holding the values
> > in ascending order at store time is faster than sorting the values at read
> > time.
> 
> Are you asking why is a bitmap used here, as opposed to a tree?

No. I'm OK with "segments using trees" + "offsets using bitmaps".

>                                                                  It's
> not just store versus read. There's also the issue that memory can get
> highly fragmented, if it is, the number of 1s is potentially very high.
> A bitmap can use as little as 1 bit per value, it is hard to beat in
> this respect.
> 

I'm asking whether we really need to invent a new library module (i.e.
PATCH 1/7 + PATCH 2/7 + PATCH 3/7) for virtio-balloon compared to mine.

What virtio-balloon needs is ability to

  (1) record any integer value in [0, ULONG_MAX] range

  (2) fetch all recorded values, with consecutive values combined in
      min,max (or start,count) form for efficiently

and I wonder whether we need to invent complete API set which
Matthew Wilcox and Wei Wang are planning for generic purpose.