Implementation of IO with Panama

[Radosław Smogura]

Hi Maurizio,

I hope you have a good day.

I’ve sent the allocator as PR (most for review purposes).

I think that even this one requires a lot of improvements, I think maybe it’s good idea to add a pool allocator bound to scope, which can create sub-allocator bound to new scope?

For benchmarks socket benchmarks are bit fragile as can slightly be different depending on socket buffer.

I did test with file, and I can get consistent gain around 15% especially for small reads 16b - on the link there’s wiki with benchmarks results.

The ArenaAllocator was very slow - it gave 50% of JNI I/O, and I think it can’t be used as if I wrap the InputStream I can’ require caller to open scope.


Kind regards,
Rado

> On Apr 17, 2021, at 12:12 AM, Radosław Smogura <mail at smogura.eu> wrote:
> 
> Hi Maurizio,
> 
> I think I know what's happening there - however I would like to find this in some documentation.
> 
> The link you shard shown that there's a call to __erno_loation and than move to/from address from rax so result from this call.
> 
> But I did objdump and on my Linux the errno is symbol in .tbss section (thread local one),
> 
> [ /usr/include ]
> radek at radek-ubuntu # objdump -T /usr/lib/x86_64-linux-gnu/libc-2.32.so |grep errno
> 0000000000000010 g    D  .tbss  0000000000000004  GLIBC_PRIVATE errno
> 0000000000029030 g    DF .text  0000000000000015  GLIBC_2.2.5 __errno_location
> 00000000001498d0 g    DF .text  000000000000005f (GLIBC_2.2.5) clnt_sperrno
> 0000000000149930 g    DF .text  0000000000000084 (GLIBC_2.2.5) clnt_perrno
> 000000000000006c g    D  .tbss  0000000000000004  GLIBC_PRIVATE __h_errno
> 0000000000129b60 g    DF .text  0000000000000015  GLIBC_2.2.5 __h_errno_location
> 
> D - dynamic or debugging symbol
> F - function
> g - global
> 
> I don't know internals of thread-locals and  dynamic symbols (I only would guess the  first is implemented by kernel by mapping separate page for every thread), and I know linking sometimes can be more complicated (i.e. gcc has multiversioning - something like bootstrap methods).
> 
> However I've found this
> https://lwn.net/Articles/5851/ (probably outdated)
> 
> I think that only manipulation of page map, could make this working.
> 
> Definitely it's worth of checking this on BSD and OSX, as well checking what kind of approach should be used, and how it can be determined on runtime, but it looks like errno_location looks like something which should be used.
> 
> From the other hand, I work on improving allocator, and I already see some good results, I'll send updates later.
> 
> Kind regards,
> Rado
> [announce, patch] Thread-Local Storage (TLS) support for Linux, 2.5.28 [LWN.net]<https://lwn.net/Articles/5851/>
> From:: Ingo Molnar <mingo at elte.hu> To:: linux-kernel at vger.kernel.org: Subject: [announce, patch] Thread-Local Storage (TLS) support for Linux, 2.5.28: Date:
> lwn.net
> 
> 
> 
> 
> 
> ________________________________
> Od: Maurizio Cimadamore <maurizio.cimadamore at oracle.com>
> Wysłane: piątek, 16 kwietnia 2021 16:30
> Do: Radosław Smogura <mail at smogura.eu>
> DW: panama-dev at openjdk.java.net <panama-dev at openjdk.java.net>
> Temat: Re: Implementation of IO with Panama
> 
> 
>> On 16/04/2021 15:10, Radosław Smogura wrote:
>> I could grab address for it just looking for symbol from CLinker.
>> 
>> But, AFIK, errno is thread local variable so maybe I should double check how this should be handled.
> 
> 
> Interesting reading from man page:
> 
> ```
> errno  is  defined  by  the ISO C standard to be a modifiable lvalue of
>        type int, and must not be explicitly declared; errno may be  a
> macro.
>        errno  is  thread-local;  setting  it in one thread does not
> affect its
>        value in any other thread.
> ```
> 
> this is subtle - it's an lvalue (e.g. can used e.g. for assignment) but
> is not explicitly declared - meaning it's not a variable in the proper
> sense.
> 
> At this point I'm not sure that linking to it gives the expected results.
> 
> In fact, firing up Godbolt shows that gcc uses __errno_location under
> the hood:
> 
> https://godbolt.org/z/rcMPTKxva
> 
> Maurizio
> 
>