Re: [PATCH] read-cache.c: optimize reading index format v4

From: Ben Peart <peartben@gmail.com>
To: Duy Nguyen <pclouds@gmail.com>, Junio C Hamano <gitster@pobox.com>
Cc: Ben Peart <Ben.Peart@microsoft.com>,
	Git Mailing List <git@vger.kernel.org>
Subject: Re: [PATCH] read-cache.c: optimize reading index format v4
Date: Tue, 28 Aug 2018 19:54:35 -0400	[thread overview]
Message-ID: <b3898649-47ee-adcc-9e22-68b23ffde5d1@gmail.com> (raw)
In-Reply-To: <CACsJy8B38QAW8qq-CctLJyJNaC329o6Rr1gs0kd=EkV+ARAaVw@mail.gmail.com>

On 8/28/2018 3:25 PM, Duy Nguyen wrote:
> On Mon, Aug 27, 2018 at 9:36 PM Junio C Hamano <gitster@pobox.com> wrote:
>>> PS. I notice that v4 does not pad to align entries at 4 byte boundary
>>> like v2/v3. This could cause a slight slow down on x86 and segfault on
>>> some other platforms.
>>
>> Care to elaborate?
>>
>> Long time ago, we used to mmap and read directly from the index file
>> contents, requiring either an unaligned read or padded entries.  But
>> that was eons ago and we first read and convert from on-disk using
>> get_be32() etc. to in-core structure, so I am not sure what you mean
>> by "segfault" here.
>>
> 
> My bad. I saw this line
> 
> #define get_be16(p) ntohs(*(unsigned short *)(p))
> 
> and jumped to conclusion without realizing that block is for safe
> unaligned access.
> 
>>> @@ -1898,7 +1884,8 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
>>>        struct cache_header *hdr;
>>>        void *mmap;
>>>        size_t mmap_size;
>>> -     struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
>>> +     const struct cache_entry *previous_ce = NULL;
>>> +     struct cache_entry *dummy_entry = NULL;
>>>
>>>        if (istate->initialized)
>>>                return istate->cache_nr;
>>> @@ -1936,11 +1923,10 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
>>>        istate->initialized = 1;
>>>
>>>        if (istate->version == 4) {
>>> -             previous_name = &previous_name_buf;
>>> +             previous_ce = dummy_entry = make_empty_transient_cache_entry(0);
>>
>> I do like the idea of passing the previous ce around to tell the
>> next one what the previous name was, but I would have preferred to
>> see this done a bit more cleanly without requiring us to support "a
>> dummy entry with name whose length is 0"; a real cache entry never
>> has zero-length name, and our code may want to enforce it as a
>> sanity check.
>>
>> I think we can just call create_from_disk() with NULL set to
>> previous_ce in the first round; of course, the logic to assign the
>> one we just created to previous_ce must check istate->version,
>> instead of "is previous_ce NULL?" (which is an indirect way to check
>> the same thing used in this patch).
> 
> Yeah I kinda hated dummy_entry too but the feeling wasn't strong
> enough to move towards the index->version check. I guess I'm going to
> do it now.
> 

I ran some perf tests using p0002-read-cache.sh to compare V4 
performance before and after this patch so I could get a feel for how 
much it helps.

100,000 files

Test                                  HEAD~1   HEAD
------------------------------------------------------------
read_cache/discard_cache 1000 times    14.12    10.75 -23.9%

1,000,000 files

Test                                  HEAD~1   HEAD
------------------------------------------------------------
read_cache/discard_cache 1000 times   202.81   170.33 -16.0%

This provides a nice speedup and IMO simplifies the code as well. 
Nicely done.