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From Greg Ames <grega...@apache.org>
Subject Re: gettimeofday calls
Date Wed, 29 Jan 2003 15:41:21 GMT
MATHIHALLI,MADHUSUDAN (HP-Cupertino,ex1) wrote:
> I had a couple of inputs here : I was talking to our specweb person, and he
> had the following views :
> 
> 1. most modern day os'es cache the files, and not do a disk io for every
> single file request. (duh !!.)

yep.  Yesterday I powered up wimp for the first time in ages and did a 
mini-SPECweb experimental run in preparation for fiddling with the stat() in 
mod_specweb99.  I got really horrible results at first and couldn't figure out 
what was wrong.  It turned out that I just needed to warm up the kernel's disk 
cache more.  The results got 50% better after an hour or so.

> 2. when doing writes, do a 64M block writes, instead of write to disk every
> time.. (Lazy write)

I would hope a smart file system/kernel would take care of that for us.

> 3. caching the fd's would be more than sufficient (than caching the
> contents). 

yep, it actually would be better for big files because we can do sendfile.  If 
your NIC has hardware checksum, a smart kernel/device driver can just do DMA 
from the file cache.  But how many fd's can you cache?

> 4. on hp-ux, eliminating the stat/fstat would not make a lot of difference..
> I dont know about other os'es - but, based on his logic, since the fd for
> that file is already available, fstat should not take a lot of time.

Some of my buddies in IBM's Linux Technology Center who run SPECweb99 say that 
one of the things that inhibits our SMP scalability with out-of-the-box Linux 
kernels is contention on the dcache spinlock.  That comes into play every time 
you do a syscall and pass a file path that the kernel has to walk.  So open()s 
and stat()s are the prime suspects.

I'm wondering why we need the stat() at all in mod_specweb99?  It looks like the 
only thing we use from it is s.size.  But IIRC the sendfile() syscall is happy 
if you give it a size of zero, which means send the whole thing.  This obviously 
needs to be tested to see how our code reacts to EAGAIN + size == 0.  If we 
can't get rid of it altogether, I would prefer to use fstat() a.k.a. 
apr_file_info_get()

The LTC guys use a "dcache RCU (read-copy-update)" patch that eliminates the 
spinlock contention, but I doubt if a high percentage of our users are willing 
to build custom kernels.  The kernel still has to walk the file path, lock or no 
  lock.

> 5. Another interesting question : why do we need the poll everytime ?.
> Instead do a accept, and if there's no request available, accept would
> return EWOULDBLOCK. 

If your box supports SINGLE_LISTEN_UNSERIALIZED_ACCEPT, you shouldn't have any 
polls before the accepts, barring bugs.  SPECweb99 does a lot of keepalive 
requests, and we poll for new data between requests on the same connection. 
That's probably what you're seeing.

Greg


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