Re: [PATCH] read-cache: increase write buffer size

From: Pranit Bauva <pranit.bauva@gmail.com>
Date: Mar 19, 2016, 10:25 UTC
Message-ID: <CAFZEwPMtv0JF+Q_1P53NB2oxH-XJXGzmiUsOxk0KBaa+AJE+vA@mail.gmail.com>
In-Reply-To: <56ED0863.3020709@web.de>

On Sat, Mar 19, 2016 at 1:35 PM, Torsten Bögershausen <tboegi@web.de> wrote:

Show 31 quoted lines

> On 2016-03-19 02.19, David Turner wrote:
>> Each write() has syscall overhead, and writing a large index entails
>> many such calls.  A larger write buffer reduces the overhead,
>> leading to increased performance.
>>
>> On my repo, which has an index size of 30m, this saves about 10ms of
>> time writing the index.
>>
>> Signed-off-by: David Turner <dturner@twopensource.com>
>> ---
>>  read-cache.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/read-cache.c b/read-cache.c
>> index d9fb78b..09ebe08 100644
>> --- a/read-cache.c
>> +++ b/read-cache.c
>> @@ -1714,7 +1714,7 @@ int unmerged_index(const struct index_state *istate)
>>       return 0;
>>  }
>>
>> -#define WRITE_BUFFER_SIZE 8192
>> +#define WRITE_BUFFER_SIZE 131072
>>  static unsigned char write_buffer[WRITE_BUFFER_SIZE];
>>  static unsigned long write_buffer_len;
>>
>>
> Do I read that right, it saves 10 milliseconds ?
> What happens to small system (like Raspberry PI), when you
> want 128K write buffer ?
> Could the buffer size be turned into a makefile variable, defaulting to 8192 ?

Maybe for all ARM devices not just Raspberry Pi.

There is one way, though its not that good. This is just a sample code. I am unsure about if using bash in makefile is a good idea

in Makefile,
uname -a | grep "arm" >out
if [ -s out ]; then
    export ARM=arm
fi
rm out

in read-cache.c,
#define ARCH (getenv("ARCH"))
#ifdef ARCH,
    #define WRITE_BUFFER_SIZE 128000
#endif