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From: Matthew Wilcox <willy@infradead.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
Subject: Re: [RFC] iomap: Remove indirect function call
Date: Tue, 31 Mar 2020 06:02:52 -0700	[thread overview]
Message-ID: <20200331130252.GA21484@bombadil.infradead.org> (raw)
In-Reply-To: <20200331074628.GA9872@infradead.org>

On Tue, Mar 31, 2020 at 12:46:28AM -0700, Christoph Hellwig wrote:
> On Sat, Mar 28, 2020 at 08:51:56AM -0700, Matthew Wilcox wrote:
> > By splitting iomap_apply into __iomap_apply and an inline iomap_apply,
> > we convert the call to 'actor' into a direct function call.  I haven't
> > done any performance measurements, but given the current costs of indirect
> > function calls, this would seem worthwhile to me?
> 
> Hmm.  Given that emount of compiler stupidity we are dealing with did
> you at least look at the assembly output to see if this actually removes
> the indirect call?  I wouldn't be quite sure.

If it does get inlined, then the compiler does it:

     b9d:       e8 ae fe ff ff          callq  a50 <iomap_page_mkwrite_actor>

If not, then the compiler emits a function in each file called iomap_apply
which makes an indirect call.  So s/inline/__always_inline/ in the original
patch.

before:
   text	   data	    bss	    dec	    hex	filename
   5314	   4648	      0	   9962	   26ea	fs/iomap/trace.o
   1050	     72	      0	   1122	    462	fs/iomap/apply.o
  17316	    636	    224	  18176	   4700	fs/iomap/buffered-io.o
   4773	     76	      0	   4849	   12f1	fs/iomap/direct-io.o
   1335	     28	      0	   1363	    553	fs/iomap/fiemap.o
   1928	     28	      0	   1956	    7a4	fs/iomap/seek.o
   1394	      8	      0	   1402	    57a	fs/iomap/swapfile.o

after:
   text	   data	    bss	    dec	    hex	filename
   5314	   4648	      0	   9962	   26ea	fs/iomap/trace.o
    722	     72	      0	    794	    31a	fs/iomap/apply.o
  18784	    636	    224	  19644	   4cbc	fs/iomap/buffered-io.o
   5169	     76	      0	   5245	   147d	fs/iomap/direct-io.o
   2093	     28	      0	   2121	    849	fs/iomap/fiemap.o
   2467	     28	      0	   2495	    9bf	fs/iomap/seek.o
   1664	      8	      0	   1672	    688	fs/iomap/swapfile.o

33110 to 36213 bytes of text.  So not free.  Worthwhile?

      reply	other threads:[~2020-03-31 15:53 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-28 15:51 [RFC] iomap: Remove indirect function call Matthew Wilcox
2020-03-31  7:46 ` Christoph Hellwig
2020-03-31 13:02   ` Matthew Wilcox [this message]

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