On Thu, Mar 20, 2014 at 2:35 PM, Karsten Blees <karsten.bl...@gmail.com> wrote:
> Am 20.03.2014 17:08, schrieb Stefan Zager:
>> Going forward, there is still a lot of performance that gets left on
>> the table when you rule out threaded file access.  There are not so
>> many calls to read, mmap, and pread in the code; it should be possible
>> to rationalize them and make them thread-safe -- at least, thread-safe
>> for posix-compliant systems and msysgit, which covers the great
>> majority of git users, I would hope.
> IMO a "mostly" XSI compliant pread (or even the git_pread() emulation) is 
> still better than forbidding the use of read() entirely. Switching from read 
> to pread everywhere requires that all callers have to keep track of the file 
> position, which means a _lot_ of code changes (read/xread/strbuf_read is used 
> in ~70 places throughout git). And how do you plan to deal with platforms 
> that don't have a thread-safe pread (HP, Cygwin)?
> Considering all that, Duy's solution of opening separate file descriptors per 
> thread seems to be the best pattern for future multi-threaded work.

Does that mean you would endorse the (N threads) * (M pack files)
approach to threading checkout and status?  That seems kind of
crazy-town to me.  Not to mention that pack windows are not shared, so
this approach to multi-threading can have the side-effect of blowing
out memory consumption.  We have already had to dial back settings for
pack.threads and core.deltaBaseCacheLimit, because threaded index-pack
was causing OOM errors on 32-bit platforms.

Cygwin (and MSVC) should be able to share a "mostly" compliant pread
implementation.  I don't have any insight into NonstopKernel; does is
really not have a thread-safe pread implementation?  If so, then I
suppose we have to #ifdef NO_PREAD, just as we do now.

I realize that these are deep changes.  However, the performance of
msysgit on the chromium repositories is pretty awful, enough so to
motivate this work.

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