I tested the your string port version and I also wrote a "string-append" 
version of the xml reader and they are both slower by about 10-15% on my 
machine, when compared to the current read-xml implementation which uses 
`list->string`.  It looks like `list->string` is not the bottleneck here.

There are some small improvements that can be made from micro 
optimizations.  For example, I changed `name-char?` to not use 
`name-start?` but instead check for all chars, and I also changed 
`lex-name` to construct the list in reverse and use `(list->string (reverse 
chars))`, plus I reordered the cond condition to check the common case 
first (that the next character is a name-char? and not a 'special one).  
However, this resulted in only about 5-10% speed improvement, nowhere near 
where the 4 time speedup when using `sxml`, as reported by John.

In the end, it may well be that speeding up `read-xml` can only be done by 
these types of micro-optimizations.  Another thing I looked into was the 
"pattern" used for reading: all the `read-xml` code will use the pattern of 
"peeking" the next character, deciding if it is good, than reading it.  
This is much slower than just reading the characters directly.  These are 
the results from just reading in a 14Mb XML file:

    read-char only:  cpu time: 312 real time: 307 gc time: 0
    read-char-or-special only:  cpu time: 750 real time: 741 gc time: 0
    peek-char than read-char:  cpu time: 1234 real time: 1210 gc time: 0
    peek-char-or-special than read-char-or-special:  cpu time: 1688 real 
time: 1690 gc time: 0

Using this code:

(define file-name "your-test-file-here.xml")

(printf "read-char only~%")
(collect-garbage 'major)
 (call-with-input-file file-name
   (lambda (in)
     (let loop ([c (read-char in)])
       (if (eof-object? c)
           (loop (read-char in)))))))

(printf "read-char-or-special only~%")
(collect-garbage 'major)
 (call-with-input-file file-name
   (lambda (in)
     (let loop ([c (read-char-or-special in)])
       (if (eof-object? c)
           (loop (read-char-or-special in)))))))

(printf "peek-char than read-char~%")
(collect-garbage 'major)
 (call-with-input-file file-name
  (lambda (in)
    (let loop ([c (peek-char in)])
      (if (eof-object? c)
            (void (read-char in))
            (loop (peek-char in))))))))

(printf "peek-char-or-special than read-char-or-special~%")
(collect-garbage 'major)
 (call-with-input-file file-name
  (lambda (in)
    (let loop ([c (peek-char-or-special in)])
      (if (eof-object? c)
            (void (read-char-or-special in))
            (loop (peek-char-or-special in))))))))


On Monday, June 29, 2020 at 5:30:43 AM UTC+8 rmculp...@gmail.com wrote:

> Thanks Alex for pointing out the use of list->string. I've created a PR (
> https://github.com/racket/racket/pull/3275) that changes that code to use 
> string ports instead (similar to Hendrik's suggestion, but the string port 
> handles resizing automatically). Could someone (John?) with some large XML 
> files lying around try the changes and see if they help?
> Ryan
> On Sun, Jun 28, 2020 at 9:56 PM Neil Van Dyke <ne...@neilvandyke.org> 
> wrote:
>> If anyone wants to optimize `read-xml` for particular classes of use, 
>> without changing the interface, it might be very helpful to run your 
>> representative tests using the statistical profiler.
>> The profiler text report takes a little while of tracing through 
>> manually to get a feel for how to read and use it, but it can be 
>> tremendously useful, and is worth learning to do if you need performance.
>> After a first pass with that, you might also want to look at how costly 
>> allocations/GC are, and maybe do some controlled experiments around 
>> that.  For example, force a few GC cycles, run your workload under 
>> profiler, check GC time during, and forced time after.  If you're 
>> dealing with very large graphs coming out of the parser, I don't know 
>> whether those are enough to matter with the current GC mechanism, but 
>> maybe also check GC time while you're holding onto large graphs, when 
>> you release them, and after they've been collected.  At some point, GC 
>> gets hard for at least me to reason about, but some things make sense, 
>> and other things you decide when to stop digging. :)  If you record all 
>> your measurements, you can compare empirically the how different changes 
>> to the code affect things, hopefully in representative situations.
>> I went through a lot of these exercises to optimize a large system, and 
>> sped up dynamic Web page loads dramatically in the usual case (to the 
>> point we were then mainly limited by PostgreSQL query cost, not much by 
>> the application code in Scheme, nor our request&response network I/O), 
>> and also greatly reduced the pain of intermittent request latency spikes 
>> due to GC.
>> One of the hotspots, I did half a dozen very different implementations, 
>> including C extension, and found an old-school pure Scheme 
>> implementation was fastest.  I compared the performance of the 
>> implementation using something like `shootout`, but there might be 
>> better ways now in Racket. https://www.neilvandyke.org/racket/shootout/  
>> I also found we could be much faster if we made a change to what the 
>> algorithm guarantees, since it was more of a consistency check that 
>> turned out to be very expensive and very redundant, due to all the ways 
>> that utility code ended up being used.
>> In addition to contrived experiments, I also rigged up a runtime option 
>> so that the server would save data from the statistical profiler for 
>> each request a Web server handled in production.  Which was tremendously 
>> useful, since it gave us real-world examples that were also difficult to 
>> synthesize (e.g., complex dynamic queries), and we could go from Web 
>> logs and user feedback, to exactly what happened.
>> (In that system I optimized, we used Oleg's SXML tools very heavily 
>> throughout the system, plus some bespoke SXML tools for HTML and XML.  
>> There was one case in which someone had accidentally used the `xml` 
>> module, not knowing it was incompatible with the rest of the system, 
>> which caused some strange failures (no static checking) before it was 
>> discovered, and we changed that code to use SXML.)
>> -- 
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>> .

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