Re: Degenerate file access patterns

[Han-Wen Nienhuys]

the repeating syscall is the read, on the same file descriptor. If
fontconfig failed a cache, you'd more likely see

 open() = 6
 read(6, .. )
 close(6)
 open() = 6
 read(6, .. )
 close(6)

I think.

I'll have a look again tonight. Maybe I can attach a debugger (my
machine has long lost the ability to compile lilypond. Sigh.)

On Tue, Mar 21, 2017 at 5:00 AM, Werner LEMBERG <address@hidden> wrote:
>
>>> "The culprit is that the lilypond binary has a bit sub-optimal file
>>> access pattern (opening the same file thousands of times and
>>> reading it byte by byte, causing a syscall flood - nearly 500K
>>> lseek and read operations).  On a local machine, because of this
>>> issue, it will spend about 1s in I/O syscalls, which is half of the
>>> total execution time. This currently does not play nice with our
>>> systems, getting it from 1s to over half a minute."
>
> Interesting, I don't get this behaviour on my openSuSE GNU/Linux box.
>
>> The font support is reading the same section of some font file over
>> and over again.
>>
>> $ cat f.ly
>> { c }
>> $ strace -e trace=open,read lilypond f.ly  >& log ; grep OTTO log|wc
>>     992    5953   90234
>
> On my box this is
>
>   1       6      88
>
>> $ grep OTTO log|head -10
>> read(6, "OTTO\0\r\0\200\0\3\0PCFF
>> \364\24\241\262\0\0\t\334\0\0\373PFFTM"..., 4096) = 4096
>> read(6, "OTTO\0\r\0\200\0\3\0PCFF
>> \364\24\241\262\0\0\t\334\0\0\373PFFTM"..., 4096) = 4096
>> read(6, "OTTO\0\r\0\200\0\3\0PCFF
>> \364\24\241\262\0\0\t\334\0\0\373PFFTM"..., 4096) = 4096
>>
>> the number of calls is apparently proportional to the number of glyphs
>> in the file:
>>
>> $ cat f.ly
>> \repeat unfold 100 { c }
>>
>> $ strace -e trace=open,read lilypond f.ly  >& log ; grep OTTO log|wc
>> 40929  245575 3724501
>
> For me, it is again
>
>   1       6      88
>
>> Werner may have better hunches than I which code is really
>> responsible for this.
>
> Maybe a problem with fontconfig?  Where is the location of
> fontconfig's database of available fonts on your system?  This must be
> created in advance so that lilypond can access it.  If it is missing,
> fontconfig tries to build it (which makes e.g. the first invocation of
> lilypond very slow on a Windows box since there is no global
> fontconfig database).
>
> Is it possible that fontconfig always fails so that it tries again and
> again?
>
>
>     Werner



-- 
Han-Wen Nienhuys - address@hidden - http://www.xs4all.nl/~hanwen